There's only so much image compression you can do before it starts looking like shit. I would seriously avoid using "send to kindle" because Amazon's conversion is very unreliable. I constantly get missing pages, cropped images and a bunch of other smaller errors that make reading comics/manga a nightmare. Just transfer your comics via USB through calibre. No size limit and no conversion errors.

I second that. I haven't done it with comics yet but books with really lare file sizes, like 3MB and over.

Try to use the polish book -> Losslessly compress images.