Looks like what it is doing, and not 100% sure why, but it is changing the scale of the bitmap. The Bitmaps will always be tile, they are only so big, but on the left, the tiles are simple scaled to a smaller size. I know when I make my maps, I tend to make my walls etc, as simple lines (File Style SOLID, and usually a color I would never use in a map like MAGENTA), once I like how they are looking, I use the MULTIPOLY Tool to combine them into a single entity, and then apply the new BITMAP Fillstyle to that.

See example here: Click image for larger version.  Name: Example.png  Views: 54  Size: 42.4 KB  ID: 34179

Now, the only question I have with your image is, Is that a screen shot from the actual software, or is that a compilation you made? Reasonis I have found that the BITMAPs usually apply to the map as a whole (i.e. you can't have a scaled 10x10 on one layer and a 20x20 of the same bitmap on another layer in the same map)