Board Thread:Questions and Answers/@comment-31147727-20190626173506/@comment-14267015-20190626185041

That's an interesting question. After giving it some thought and checking around the wiki I have a couple theories. First is that many of Ben's aliens mature much faster than humans do. While all of Ben's transformations are the equivalant age he is, we have seen at least one major example of this in the form of Attea. When we first see her in AF she seems to be about 8, 9, or maybe 10 years old, by the time of OV (about 1 year later canonically) she has obviously. . . grown. So it stands to reason other aliens can go through similar growth spurts. Plus, the aliens of the first Ben 10,000 we are introduced to displayed changes we have to assume are the result of continued aging.

The second theory I came up with involves the state the Omnitrix puts the transformation in. That meaning all of the aliens Ben has become are in a state of their specie's prime condition. To put it in easier to understand terms they are like the Captain America, ie Super Solder, version of their race. This is best seen in OV when Fourarms beats Looma in a fight, male Tetramands are typically weaker than the females. Bullfrag is another example as, despite most Incursions that appear being small and frail, he is obviously not. Other forms may have become similarly taller and more powerfully built despite their age. Personally, I think it is mostly likely a combination of both ideas.