[popcornmix]

Just for background, a 1280x720 texture will use 4MB of graphics memory. A 1920x1080 texture will use 8MB of graphics memory.
If a skin then tries to pack a number of these in (e.g. for a wall view) you are talking about significant graphics memory usage.
E.g an 8x4 wall of 1080p images will require 256M of graphics memory for just that view. Textures persists for 5 seconds after they disappear from screen,
so if you are scrolling a view like that you are talking about gigabytes of graphics memory required.

[Hitcher]

I think you might be confused into thinking that using <imageres>1080</imageres> makes the bookmark thumbs 1920x1080 whereas they're actually only 540x303. Using the default <imageres>720</imageres> makes them 360x202 which is half that of 'normal' thumbs.

[popcornmix]

It does sound like the bookmarks should have their own setting, rather than relying on half someone else's setting.
If you do increase <imageres> purely to get higher quality bookmark images, then you will have the (full) increased memory usage for posters and other artwork.