They're as-is art from the (long-dead) mobile game Psychic Puzzle! Which, while a canon product, doesn't always match the canon of anime/manga either by timing (choosing color schemes for characters that hadn't been in the anime yet, like the Minegishi/Hatori/God Ekubo) or to make a visual gag.

In canon Teru is usually between well-defined to ripped as hell. There's not a lot of the others besides Mob that can be compared — The onsen artwork doesn't show them below chest, the cancelled summer OVA stuff has Ritsu and the adults clothed (Teru's well-defined, Mob's soft), the artbook shows only some of Mob's stomach (soft), the summer omake is doodlier than the manga and can't be referenced.

(Going to personal beliefs here: Ritsu's good at school sports, but he's not interested in exercise or muscles like Mob is, so while a fun gag on the whole "Mob is not as buff as he dreams and everyone else is more built" I think he's only a little more defined, max. Definitely not more than Reigen who's an actual adult and surprisingly defined when ONE drew him.)

But yeah TL;DR with the evidence we have Teru's among the most muscular, Reigen's semi-defined and Mob's the softest (Ritsu's a mystery)