In the Classic Editor block, the header text sizes (and line heights) do not match the header text sizes used in the rest of the editor. These should be identical.
Classic Editor:

Gutenberg Header Blocks:

Closed #8069 to consolidate here and noting that #8069 makes a case for updating to font sizes that work out to values in pixels without decimals in order to improve consistency between browsers since each may round slightly differently. For example:
2.5em x 16 = 40px
Cool 鈥斅營've updated the title to make it a bit more broad in that case.
Please notice in #8069 there's some more detailed information that is important to correctly address this issue. Also, avoiding rounding should be considered also for paragraphs, see example and screenshots on https://github.com/WordPress/gutenberg/issues/8069#issuecomment-407006362
Hi @kjellr it seems like to solution to this is update the classic block styles to match the rest of the editor. Could you confirm if that's the case?
@jorgefilipecosta Yes, that's a good first step. I think we'd also wanted to address the fraction-of-a-pixel rendering caused by our type scale (as noted in #8069 too), but that can be handled separately.
For now, it's definitely weird that a h2 in the classic block isn't the same size as a h2 outside of it for instance. So fixing that would be great.
Most helpful comment
Please notice in #8069 there's some more detailed information that is important to correctly address this issue. Also, avoiding rounding should be considered also for paragraphs, see example and screenshots on https://github.com/WordPress/gutenberg/issues/8069#issuecomment-407006362