It seems that the Marketing App editor generates some really bloated HTML markup to the email template which bumps up the email sizes substantially. For example, it seems to generate id attributes for almost all elements and the ids are really long, like this:
id="FullPage_1d3cc1695917c1626775535571FullPage_a148414ef84781627280053151FullPage_dd353f4241b5b1627292079924FullPage_66f3908c749a81627293499993FullPage_8286a35e1d2891627294308094FullPage_10ec07b381ea21627301605725FullPage_0419a012ba68b1627302484538FullPage_0ffd78e0b601e1627303362562"
Those ids alone make the email size grow from 90KB all to way to 175KB, which is far beyond the 128KB limit. I wonder if there's a way to get rid of "junk" and/or minify the HTML before sending the email?