Two enlargements, not one
Scaling a cartoon is a division and it goes wrong when the drawing and the opening are different shapes. The page prints both scale factors so the mismatch is visible: 9 by 12 into 18 by 24 is exactly 2 by 2, but 9 by 12 into 18 by 26 is 2 by 2.167, and enlarging on one setting will not give you both.
When they differ you have three options and only one of them is any good. Enlarge to fill both directions and the design distorts, which on anything with a circle in it is immediately obvious. Enlarge uniformly at the smaller factor and you are left with a strip of empty panel. Or redraw the border to absorb the difference and keep the interior at one scale, which is what borders have always been for.
The heart, and why it only ever makes the panel bigger
Where two pieces of glass meet in a leaded panel, they do not meet. The heart of the came sits between them, and the cartoon line is drawn down the middle of that heart. So each piece stops half a heart short of the line on every edge, and a piece bounded by lines on all four sides comes out one full heart smaller in each dimension.
A 3 by 4 piece on the cartoon, at a 2x enlargement, is 6 by 8 at full size. With a 1/16 inch heart it gets cut at 5-15/16 by 7-15/16. That is a small correction on a piece that size and it looks ignorable, which is the trap, because it is not the piece that matters — it is the row.
Six pieces across a panel cross six hearts: five between the pieces, and half a border heart at each end that adds up to a sixth. Six sixteenths is three eighths of an inch. Cut every piece to the line and the panel comes out three eighths wide, and eight pieces down means it comes out a half inch tall as well. Every line adds. Nothing subtracts. There is no cancelling error to save you, which is why the overshoot is a function of how finely the panel is cut rather than how big it is.
Small pieces lose the most
The allowance is a fixed width, so what it costs a piece depends entirely on how big the piece is. A sixteenth off a six inch dimension is one percent. A sixteenth off a half inch dimension is twelve and a half. A design full of small pieces is a design where the cut line is meaningfully different from the drawn line on nearly every piece, and cutting fat with the intention of grinding back is a plan that turns into an evening at the grinder.
This is also why traditional pattern shears cut three strips at once and throw the middle one away — the width of that discarded strip is the heart, and the shears take it out of the pattern automatically so the cut line never has to be worked out piece by piece. If you are cutting patterns with scissors, you are doing by eye what the shears do by construction.
Three different sizes and only one of them is the one you want
A leaded panel has an outside dimension, a glass dimension and a sight dimension, and they can be three quarters of an inch apart on a small panel. The outside is what has to fit the opening. The glass edge sits in from that by however far the closed back of the border came stands past it. The sight size — the glass you can actually see — sits in again by however much came covers the face.
On the defaults the glass is an eighth of an inch narrower than the 18 inch panel, and the visible glass is another three eighths narrower again — half an inch of the width gone before any glass shows. Design to the sight size and the panel will not reach the opening. Design to the outside and the image is half an inch smaller than you drew. Decide which one the client cares about before the copier, not after the came is on.
Copper foil is not exempt, just quieter
Foiled pieces very nearly butt, but two thicknesses of foil still separate them, which is a few thousandths of an inch. Across sixty joints that is a fraction of an inch and it is real, though it is dwarfed by the other thing foil does: every piece gets ground to fit its neighbours, so the errors go into the grinder rather than into the panel size. That works beautifully until the last row, where there is nowhere left for the accumulated difference to go.
The practical version is the same for both methods. Lay the cut pieces out dry on the full-size cartoon before anything is assembled, and check the outside dimension against the opening while there is still time to grind rather than remake.
Questions people ask
Why did my stained glass panel come out too big?
Almost always because the pieces were cut to the cartoon line instead of inside it. The came heart takes up room between every pair of pieces, so each piece has to lose half a heart on each edge. That is invisible on one piece and cumulative across a row: six pieces across a panel cross six hearts, and at a sixteenth of an inch each that is three eighths of an inch of extra width. It only ever adds, so the panel always ends up bigger and never smaller.
How much do I take off each piece for the came?
Half the heart thickness off every edge, which means the full heart off each overall dimension for a piece surrounded by other pieces. Measure the heart on the came you actually have — callipers, not a catalogue. Traditional pattern shears do this automatically by cutting three strips and discarding the middle one, and the width of that discarded strip is the heart. Cutting patterns with ordinary scissors means doing the same job by eye.
What if my drawing is a different shape from the opening?
Then there is no single enlargement that works, and the page prints both scale factors so you can see how far apart they are. Enlarging to fill both directions distorts the design, which shows up instantly on anything circular. Enlarging uniformly at the smaller factor leaves an empty strip. The usual answer is to keep the interior at one scale and redraw the border wider on one axis to absorb the difference.
What is the difference between panel size and sight size?
The panel size is the outside of the border came and it is what has to fit the opening. The glass dimension is smaller by however far the closed back of the came stands past the glass. The sight size is smaller again by however much came covers the face of the glass — that is what you can actually see. On the defaults here the glass is an eighth narrower than the 18 inch panel and the visible glass is three eighths narrower again, so half an inch of width disappears before any glass shows. Deciding which one matters is a step before the copier.
Does the allowance matter for copper foil?
Less, but not zero. Two thicknesses of foil separate the pieces where they meet, which is a few thousandths and accumulates across a panel the same way the heart does. The larger effect is different: on a foiled panel every piece is ground to fit its neighbours, so errors go into the grinder rather than into the size. That absorbs a lot, right up until the last row, where the accumulated difference has nowhere to go.