I tried to view "The New Oxford American Dictionary", a 28-MB MOBI file. It eventually timed-out with errors. One of the errors was a memory error.
Possibly Viewer will work with large files if PC has enough RAM.
I have several EPUB's in the 13-MB range that work fine. Don't have any bigger EPUB's to test. So ... problem could be some combination of file size, MOBI format, dictionary format or RAM!!
PS: Neat signature!