I'm with you....I was shocked when I put my first 8 frames in my first box. As a matter of fact, probably more shocked than you, or should I say scared. Remember, I built my own boxes, so I was totally convinced for a moment that I had totally botched my measurements and built the wrong size boxes. LOL!
With my setup, I've got maybe a little less room than you, but basically 5/8" on each side, which seems quite significant. Trust me, it's not. You'll be glad to have the space to work the hive. I'm a newbie also but the approach I've been taught and have been taking is to just keep your frames pushed together and centered....leaving equal free space on each side of the box. Somehow once you add bees, drawn comb, propolis, and maybe a SHB jail or two....that extra space seems to reduce significantly.