I might be wrong, but my reading of why Kostya chose to have certain value to which to resize the
inline images is because the usual purpose of them is to see "inline", as a part of the webpage-like formatted message.
I might agree or disagree about what that size should be, but I understand (and agree) with the rational behind it: If you are creating a "webpage-like" message, the intent for it is so it would be read on the screen. (I've never known what that size is limited to, as I very seldom use that feature, if at all. My personal choice from the user's perspective would be probably not to exceed some 500-600 pixels on each side.)
If you need to send a full-size image (e.g. for printing, looking at details, or for pixel-peeping), you'd send it as an attachment.
Playing a devil's advocate: what about when one wants to print an e-mail with the high quality inline image. For those purposes you really want to use a more robust "instrument", such as MS-Word or PDF document. Why? -- Among other reasons, the actual layout of the formatted e-mail can differ in different e-mail clients anyway.
Personally, I (and several of my colleagues) hate when one sends 6-10 Mpixel inline image. So, in a way, Kostya's app is doing a service to the society by helping to limit that thoughtless abuse and "waste of electrons"
(when other forms of education don't help).