git gc ineffective

Hello,

I am running git 2.29.1

I noticed I am running out of disk space, and one repository taking up
about 38G. Did git gc --aggressive, and the used space *raised* to 42G,
and git would report it does gc after every commit. Did gc without
--aggressive, and the used space almost *doubled* to 75G (about 1G is
checkout and some utracked files).

The expected size is about 20G. Is there some way to debug gc to figure
out what went wrong?

Thanks

Michal