fruit bats are the biggest bats

not GP but I think that was the point.

also, volume grows as the cube of linear dimensions which also puts an upper limit on size, as wing surface area only grows as the square (not sure what/how lift grows relative to)

Plants aren't particularly calorie-dense. Meat, on the other hand...

this is almost in "not even wrong" territory, but for the fact that autotrophs are definitionally the entry point for abiotic energy into edible calories for animals, and the observation that the largest terrestrial megafauna are herbivorous.

bamboo is not calorie dense to humans, because we've lost the ability to digest most of it, but pecans are absolutely more calorie dense than even fatty beef.

all else being equal, an ideal carbohydrate source is more calorically dense than an equivalent ideal lean protein source due to the balance in the thermic effect of food between the two. most mammals outside the obligate carnivores are really well optimized for getting calories from plants— this is why we have amylase in our saliva.

Look at great apes. Large land mammals in general. (Apes came to mind specifically because they usually eat fruit)

Are you aware you switched "fruit" for "plant" there?

Fruits want to be eaten, Veggies don't.