Earlier quoted context omitted.
First, it is not a browser. You actually do commits, merges and manipulate branches. The size is that big because Gitbox is bundled with official Git binaries so you don't have to install anything else. I will see how to bring the size down in the next versions. Anyway, this fact does not affect the performance and does not eat much memory.
so is that better than gitx?
Gitbox is better because it is a pleasure to use. There are tons of little details which matter in a daily work. For instance, when you switch a branch and try to commit to it, the branch name will be highlighted to remind you that the branch was switched since last commit. If you forgot to switch back, you just cancel the prompt. This little thing instantly eliminates 50% of use cases for "git reset --hard" or "git cherry pick" and makes you happier.
Also, Gitbox fetches remote commits automatically so see that you have to pull something: this eliminates unnecessary merge commits.