This story hails from Outer Space #23 (as always available on Golden Age Comics).
Let's get into it.
We join an intrepid spaceship crew on a routine patrol...
Of course, we can expect the patrol will become much less routine shortly.
One of the things I really love about this old story is the spaceship design it features. They're charming old fake submarines, aren't they?
It is not long at all before crazy stuff goes down. They detect a "large, active body." They have no idea how literal that statement truly is, as they don't get a good look at what's coming until much later...
Look, it's Galactus's obnoxious younger cousins!
Well, no, not really.
The giant that grabbed the ship stuffs it in his pocket, and the two wander off. The ship's crew considers blasting their way out, but decide to call for help first. This is a very unusual show of foresight on the captain's part for stories like this.
Seeing just what the other ship is trapped in, they send a message to them that it ought to be easy to get out. And so it is:
The patrol ships, deciding that these giant aliens might possibly disrupt shipping (ORLY?!), open fire, and manage to chase them off.
They return to their home planet, where we learn...
...that they're little kids. Which actually explains their behavior pretty well-if you were a thousand foot tall space giant child, wouldn't you snatch at passing spacecraft and maybe stick them in your pocket? (Also, their mother is surprisingly attractive.)
Outer Space and its close relatives regarded the kind of story that this one is, the one with a bizarre reveal related to scale, to be its own genre, and had rather a lot of them. This might well be the best such story, mostly because it's just plain funny.