I do the same - and I'm puzzled how little that animal eats, compared to other animals -, with exactly 2x per week bugs (and not much), 1 to even 2 times a week fasting (normally he won't eat for 1 - 2 days per week on his own), extra veggies most days, golliwog available always.
His weight is 560 g. He isn't chubby, and I bet I myself don't lean towards "a bit chubby is still lean" (also not with myself, so for sure also not with pets).
A thought: Is it maybe the case that the wild bearded dragons are smaller, as in: shorter, and maybe also narrower in bone structure*? Similar to e.g. like in some birds e.g. budgerigar where the wild form is just significantly smaller than the domesticated form even in those showing the "wild" colors?
*I'm aware that an overweight individual, being it pet or human, is normally not heavier just as the shoulder bones are wider apart, rib cage is larger, hip bones wider - even as some people claim for themselves

On the other hand, however, if that's indeed the case for some individuals and already the internal dimensions are different (like in the budgerigar case: they are larger, also their bones, beak, all that), more tissue would be needed to go around that, i.e.: heavier despite lean.
I agree on this.
In the beginning I was slightly worried about having the enclosure in front of two windows, and I thought as nice as it looks, maybe I will have to use the curtains once in a while to not overheat.
The reality is: My dragon enjoys sitting under the lamps even when it's really hot outside and it heats up through the glass even more. When he's done basking, he goes away. (This is, in my opinion, also the reason why a large enclosure should be offered - for the temperature zones. A dragon sitting at the same spot for hours - mine does - and not acting "sportive" is IMHO no reason for a small enclosure.)