No it's not just you. I find it hard to explain but GitHub does have the right amount of white/empty space to make consumption of information easier. I've always hated the simplicity of GitHub's layout, but after studying their commits and branches pages for my product, I've grown to appreciate their simplicity.
It's the little things that GitHub does, that I've found makes the difference. Take GitLab's branches page here:
https://gitlab.com/gitlab-org/gitlab-ce/branches
vs
https://github.com/gitlabhq/gitlabhq/branches
By limiting the number of branches shown and by creating strong visual breaks/barriers, they make it way easier for the user to focus. Seeing a wall of branches is okay ... when that's what you want, but as a default, by organizing it the way that GitHub does, they make their branches page significantly easier to consume.
I also find the buttons too be a little too big in GitLab and they don't have enough definition to them, to help you focus on them. Creating merge requests and comparing branches is the focal point of the branches page, and they should make it easier for your eyes to lock onto them.
Like I said, it's the little things that I can't explain that I've come to respect and I don't think it's just because we are familar with GitHub.