The Suspender Avenger

This new hero sure didn't stick around long:

...at least, not in this costume.

In the transition from Ant-Man to Giant-Man, Marvel kept several elements of Ant-Man's costume, and jettisoned others. My guess is that they were trying to strike a balance--offering readers a "brand-new" character, yet needing to retain features of the character that had become familiar to readers thus far. Since Giant-Man was intended to be featured more than Ant-Man from this point on, certain things had to be left behind. Most noticeably absent was the Ant-Man helmet, which was probably seen as tying Giant-Man too much to the Ant-Man character--and since Giant-Man was going to rely on brawn more than ants, the helmet would be more distracting than functional. So Giant-Man was simply given two antennae protruding from the top of his mask. (Which ironically would have served Ant-Man just as well as a bulky helmet. But Ant-Man needed a "device" like the helmet to sell his character, whereas all Giant-Man needed was his tall size.)

As you can see, the Giant-Man character also kept the jagged design of Ant-Man's gloves and boots, as well as the coloring of his costume. Yet what I always felt was a cool design--the dark circle chest symbol (representing science? size-changing? I don't know)--was something Marvel attempted to continue with Giant-Man (albeit slightly altered, with lines extending to the shoulders and down the back), but simply discontinued after only one issue of the character's first appearance in both Tales To Astonish and The Avengers. In its place, Giant-Man was given a "suspenders" look--a step down in design, since there was nothing left in Giant-Man's costume to make him uniquely stand out in the group. Nor did small art panels (at that time) help matters, since an artist couldn't illustrate Giant-Man's size advantage to its full potential.

The transition between costumes was, dare I say, astonishing:

The "original" new Giant-Man's first and only appearances

The character after his "adjustment":

Perhaps Marvel felt that the chest symbol still tied the new character too closely to Ant-Man. In a way, I can see that--particularly on the cover of The Avengers #2, where the constraints of the cover make him seem less like a "giant" and more like a different version of Ant-Man.

At any rate, as we well know, it wouldn't be the last alteration of Giant-Man's costume. (Not by a long shot!) He even went through a few before his second departure from the Avengers:

Hey, looks like Hank even found a reason to readopt his old helmet!  Feeling he needed a "weapon" in order to pull more of his weight with the Avengers, he adapted the helmet to give it the ability to change the size of other objects by "remote control."  Initial testing proved disastrous, though, so he ended up reserving its use for himself--though he'd already earlier developed a way to change size without using gas or capsules.