That was either a translation error or the drawing style. You can see a significant difference between the two styles here:
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Vegeta was USS in some shots yet his muscles weren't big enough and looked lopsided - he was SS1 in some shots, according to the "manga", regardless of drawing style.
Trunks was USS in some shots yet his muscles weren't big enough and looked lopsided - he was SS1 in some shots, according to the "manga", regardless of drawing style.
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I think a lot of people get confused by the drawing styles--the particular style I referred to above was probably done by some of the worst artists/animators at Toei Animation; the proportions are obviously off and even though the character should've had larger muscles they were drawn stubby and flat. Although, there are some things about that style that looked to carry a certain amount of extra detail, as far as lighting goes, and other things:
*shrug*
On the "good drawing style" one (my opinion), you can also see if you've ever seen the episode, that the coloring, lighting & animation was very, very good compared to episodes that the first drawing/animating style I posted was used in.
This was obviously a misunderstanding. A lot of people used to think Trunks was SS2 here. I'd like to mention Trunks would've probably had to reach FPSS (Full Power Super Saiyan) before moving to SS2--and it's safe to assume a Saiyan couldn't move from USS to SS2. In any case, Trunks had no time or reason to go to FPSS, let alone SS2. A dead giveaway for SS2 is electricity sparks and much longer & spikier hair. Neither are present in the episode Trunks killed Cell/Androids. The most likely forms were USS1 or SS1, seeing how Trunks had learned prior that using USS2 was pretty useless and a total waste of energy. In the manga, Trunks and Vegeta were only in the SS1 form during the Cell Games, and not USS. So that may have had something to do with why you see a decrease in muscle size, somewhat.
Anyway, the drawing styles/animation vary so much in DBZ. It's hard to determine certain things.
http://www.youtube.com/watch?v=y84mzbeSuiU
All of that is from Episode: 'Hour of Temptation', where Cell begs Vegeta to allow him to become complete. That's one of TOEI's better drawn/animated episodes, in my opinion.