The skin is expecting more typical info page content, so it's leaving some "breathing room" around the box, so that the usual two-color scheme (box on background) always works. Easy to defeat with one more piece of custom CSS:
.ja-ipbox > div {
max-width: 400px;
}
The regular CSS is setting the max box width to 10px less than your stated max page width.
Funny coincidence - the only reason it looks OK to you on a phone is that the screen width on the phone is in the same neighborhood as your max page width. The info page popup gets reduced to something
less than 390px, so the image fills the box!