In Captain America: The First Avenger movie, when Steve Rogers wasn't Captain America, he looked tiny (if you have better word, please replace) but the same. Was this role also played by Chris Evans with help of computers? Or, there's a doppelganger actor who played that role?
Yes. They used digital effects to shrink Chris Evans so that he looked like a wimp.
There is an extensive write-up about the details of how it was done (with pictures) on fxguide.
For illustration, here's one of the pictures from the link of him in the original plate:
And after the digital shrinking effects:
"Leander Deeny" - this is the tiny guy. The director films him with the same hair style and same view of "Chris Evans". After he gets in the machine, he's converted an comes out as "Chris Evans" with muscles and sexy body.