In Captain America: The First Avenger, before Steve Rogers was Captain America, he looked tiny but the same. Was this role also played by Chris Evans with the help of computers? Or was there 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.
“Leander is the unsung hero of this,” Edson Williams, visual effects supervisor for Lola, told TheWrap. “He was very dedicated and he was very aware of mimicking Chris’ timing. He wasn’t trying to get his performance out there. It’s his biggest credit and it’s a role where you never see his face.”
To attach Evans' head to Deeny’s body, the filmmakers would first shoot Evans in a scene. Then they would have Deeny watch what Evans had done on video playback, so he could mimic his movements precisely. Lastly they would film a clean plate, this is a pass devoid of principal actors that allows the background to be replaced behind the shrunken Evans.
“The heady replacements were tricky, because you were taking the head of a rhinoceros and putting it on the body of a gazelle,” Williams said. “The difference in muscles, in connective tissue was so vast, that it was very difficult to make the necks match up.”
“It’s almost always Chris Evans’ adams apple.”
The body, on the other hand, you’d never know it, but at least 10 percent of the time, it belonged to Deeny.