I see a difference between the 2 pics, but I don't really know what you're saying and what you mean by "taper".
Are you just saying that given the same tabs (same number of tabs with same sites etc.) that the favicons get squished together in 75 and not in 74? If so, I would guess that just has to do with the default size of a tab changing or something and that there's no way to fix it.