A balanced fighter looks normal with an even distribution between strength, speed and stamina. Trunks' Grade III SSJ increases strength and decreases speed, while stamina may have already taken a hit from Grade II, improving just a bit with this one, perhaps. Here's an example:
[Grade II SSJ]: Strength - 625'000 | Speed - 625'000 | Stamina - 320'000. The end result: he unleashes good damage (STR * SPD).
[Grade III SSJ]: Strength - 800'000 | Speed - 390'625 | Stamina - 400'000. The end result: he has the best defense (STR * STA).
Now, I believe that a speed based Grade III may be able to unleash more damage than a strength based one, and look thinner as a result. Here's the stat distribution:
[Grade II SSJ]: Strength - 320'000 | Speed - 625'000 | Stamina - 625'000. The end result: he has a high dodging rate (SPD * STA).
[Grade III SSJ]: Strength - 400'000 | Speed - 800'000 | Stamina - 390'625. The end result: he unleashes the best damage (SPD * STR).
Last, but not the least, a stamina based Grade III may be able to dodge at a more frequent rate than a speed based one. However, what would it look like?
[Grade II SSJ]: Strength - 625'000 | Speed - 320'000 | Stamina - 625'000. The end result: he defends himself quite well (STA * STR).
[Grade III SSJ]: Strength - 390'625 | Speed - 400'000 | Stamina - 800'000. The end result: he has the best dodging rate (STA * SPD).
Conclusion: I believe that he would look a bit more chubby and not so stiff.