I’m adding this note here in case someone stumbles over this page because of the same problem i had.
My published site was also always about 25% bigger than the preview in webflow, even sizes with fixed pixel units. On all browsers too.
Then i remembered my display settings on Windows were set to 125% =)
(This operation system setting works on all browsers, but not on webflow designer or preview, which it shouldn’t of course)
Maybe this helps someone save some time puzzling…