I'm not crazy about anything mentioning 'appetite,' or 'filling,' or 'hearty' unless they really are wintery things like thick stews.
I also wouldn't use 'king-size' or 'extra-large' or whatever, even if they are, because that's just not done -- making people aware that they're eating like pigs tends to discourage them from doing it. Restaurant portions are just larger and larger on average these days, at least on this side of the pond.
If it really is exactly the same foods as the smaller one, just in more quantity, and the smaller one isn't specifically for children or seniors, then often the menu will just list the size and the price, e.g.,
Schnitzel with mushroom sauce ....................... Regular $10 ... Large $15
(or small/large, small/regular, medium/large, etc., depending on emphasis)
If it's mostly a different choice of foods, the traditional thing here is just to list one group as 'appetizers' and the other as 'entrées,' which I realize you probably can't use for BE. People with smaller appetites are just aware that they can order an appetizer as their main course. I wouldn't use 'main dishes' unless it was served family-style, in a serving dish or platter in the middle of the table, but you could say 'main courses' instead of 'entrées.'
However, perhaps to make people less self-conscious about ordering a small size, and to offer more variety/flexibility, one thing I've seen in some restaurants here recently (not chains or diners, but more the kind that have some pretension to cuisine) is 'small plates' and 'large plates.' That way people can even choose two of the small ones for more variety, sort of like tapas or whatever. That solution might help solve the problem of the AE/BE differences in the other names for courses.
'Light plates' is also okay, though it tends to sound more like diet/low-calorie/healthy/good for you, and it often tends to be things like broiled fish or grilled chicken and steamed vegetables that aren't particularly appealing. I wouldn't use 'bites' unless it really is finger-food-like appetizers.
I think there have been threads on this in the past; you might also check the archive under 'großen Hunger / kleinen Hunger.'