Well most of them are in the same image already, it's not referencing different images for each asset - they are mostly part of a large palette. As far as doodads, about half my list of doodads is vanilla hammerwatch and the other half is stuff I've imported - actually a bit less than half. The difference would be in the scaling of the assets - some 16, some 32, some 64, etc. Also the amount of doodads I have placed and such seems to be quite a lot - they are most times smaller and require more of them to fill the same space. Lots of doodads overlapping, etc.
Crashing occurs most when changing doodad layers, copying doodads, and (almost everytime) reloading resources in the editor.
I checked the .xml file size and it is just under 6MB - I thought maybe the file size was some indicator but after looking at the file sizes of the vanilla levels some are bigger.