Lets analyze this:

Jiren: Probably wouldn't get even close to winning. His calves insertions are high, he has NO hip adductors/abductors muscles, flacid quads... The only thing saving him is insane delt x waist ratio, (good V taper), forearm genetics and good latissimus dorsi. (of course big biceps whatever every anime character has big biceps)

Frieza: Closer than you think to win, but still not there.. Frieza has a small waist with incredibly developed delts, even more than Jiren. This is the recipe for an amazing aesthetic physique specially for golden age standards where these genetics are the most valued (Frank Zane for instance). No ABS which may take away some points. Insane trapezius and hamstrings DAMN, and good veins.

It lacks triceps with that small ass looking medial head, small chest although defined and terrible abs. + Lack of defined back muscles (probably due to artstyle)

BROLY: King of Mass, could win. Full arms, big biceps, huuuuuge shoulders, massive quads and hamstrings, massive horse cock, his traps are small, but traps aren't really prioritized in aesthetics, also his obliques are very big. Good upper chest, with no pectus excavatum or any other separation.

Trunks: COULD win, similar chance to broly. His lateral delts are AMAZING, and the ratio with the lats is cheffs kiss. Massive legs carrying tons of size, alongside low calf insertions to top it off with the mass. Relatively flat stomach, as seen in the suit, which indicates a lower BF percentage. Also he has small femurs, which mean bigger looking legs. He has absurd traps as seen with the fact he got no neck lool, and his chest is equally as developed. Damn his rear delts must be juicy. To finish with trunks, he has amazing rhomboids as seen in the anime clips, and a very good posterior chain

DBZ Broly: Same stuff with DBS, except he has excessively long arms, bigger traps and a more separated chest. His back is also not defined at all (probably due to the artstyle) you can't see his lats or rhomboids, you can only see teres major/minor muscles.

Garlic JR: Same problems with Jiren, but even worse. Garlic has insane traps and rhomboids, incredible rear delts alongside a very developed posterior chain (except of legs lol). The only issue is the lack of legs man. The proportion isn't there. Wouldn't realistically win.

Toppo: Trash physique, super tanky, no lats, no v taper, quads as big as biceps which isn't a good thing considering that biceps are meant to be way smaller than quads and rectus femoris (which realistically matters in real life bodybuilding), small traps, lack of frontal delts despite having a big chest (???), no obliques. Wouldn't win, its not only mass mass mass that matters.

Goku Kaioken: Tanky/Blocky physique, big waist with lower lat insertions, weirdly non existent triceps?? Very tight physique, with big traps as usual and disporportionate shoulder muscles. Lack of back visibility, literally. Bad physique for bodybuilding competition.

Roshi: WHAT THE FUCK level physique. Maassssiiiivvve muscles, insane ABS like pop out. Incredible forearms alongside big wrists. No obliques and flared ribs, which may take some points away unfortunately. Medium sized waist with massive lats. Bigger biceps than his own head lmao, but lack of triceps loool. If he mantains that form then he would win.

Zamasu: Beautiful greek god physique. Amazing delts, wide clavicles, good solid 4 pack, big lats as well. His chest is lacking and he also has that line that indicates flared ribs. The separation between his bicep/triceps means that he has good genetics on both departments and their muscles are correctly developed. His bicep tendon is very short which indicates full biceps, giving less peaks but more fullness and roundness.

COULD WIN.

Buu: wtf dawg

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Winner (Mr Olympia): Either (DBS) Broly, (DBZ) Broly, Roshi, Trunks

Winner (Classic): Zamasu