I was originally going to say “big”, but I don’t mean sales, popularity, relevance/impact, canonicity or anything like that.
I also don’t mean the size of the cart or ROM, which are objectively measurable things.
If you were asked to name Game Boy games that feel large when you play them, what would your answers be?
Once you discard the stuff I already ruled out, you can be as subjective as you want. You could think spatially, game worlds that feel large. It could be pure content measurement, like if it has 200 discrete levels or 400 different monsters to fight. The variety of things you can do might make it feel large, or the length of time it takes to do everything, or… whatever feels appropriate.
If you want to name/talk about “large” games on other handheld platforms, that’d be great too. Go wild.
I’m still thinking about it myself. I don’t think I’ve played a lot of ones that actually feel large, but plenty that feel small.
A less common answer might be Lufia GBC, which seems pretty large, at least in contrast to other RPGs on the system.
Final Fantasy IV and Front Mission would seem pretty large on the Wonderswan, but don’t seem nearly as large on the DS.