As said already, that's the point of a flat white. Anywhere that does different sized flat whites are just selling you a latte.
Lattes (for me) are way to much milk, so a flat white is ideal. Or, if I want a bigger cup, then a latte with an extra shot is the closest to doubling a flat white I've found.
And for a PP - cappuccino should be 1/3 espresso, 1/3 milk and 1/3 foam shouldn't it?