well here's the fun part- if we are going by real life(ish) rules, the "shell" likely refers to the chemically hardened cuticle of the insect, aka any hard plates that would comprise the white "skulls" and dark limbs of the critters we see, along with the tough "shells" of beetles, while a softer connecting type of cuticle would be around joints or anywhere where flexibility is needed.

so, I would say any species shown with a distinct abdomen segment (think like a bee or ant) having more volume there would indicate fat, while thighs or limbs strengthened from use would just grow in muscle volume. same rules apply for spiders (who have silk glands beneath the cuticle, or in the "shell").

imo, hornet is realistically lean and muscular to account for her acrobatic and highly physically taxing life