fix formatting for values <1000

[hugovergnes]

Fixes a small formatting issues for values below 1000. The number of significant digit would be ignored.

# Before 
print(_format_size(4.843658346, sig_figs=3)) --> 4.843658346
# After
print(_format_size(4.843658346, sig_figs=3)) --> 4.844

[meta-codesync]

@bottler has imported this pull request. If you are a Meta employee, you can view this in D91776028.