The problem code is on line 267: img {max-width: 100%;}. I can't tell if it's custom code that's been there before or if it's with the new release—my guess is it's old code. The breakpoint it's focusing is on 768px, which is why it looks like it jumps twice.
When I removed that line, the mobile version and in-betweens look great!
Check in your landing page template and landing page editor to see if it's residual code from before. If it's not on your end, it might be a bug that Marketo can squash. (You can CTRL+F "img {max-width: 100%;}" to see if it's in the template.)
Edward Unthank | Founder, Etumos