Why do you prefer bulma?
I think people really au fait with CSS prefer to add only what they need rather than serve Bootstrap entirely.
But that's all optional, Bootstrap is modular [1] [2] [3].
[1] https://getbootstrap.com/docs/3.3/customize/
[2] https://github.com/twbs/bootstrap-sass/blob/master/assets/st...
[3] https://github.com/twbs/bootstrap/blob/v3-dev/less/bootstrap...
That's a drawback. For BS3 or 4 I can just comment out what I don't need. If I do need many of the components and JS modules that Bootstrap provides OOTB, I have to write them myself. I'm using a CSS framework to avoid doing that.
Edit: And can be easily customized.