Unity crashed on reopen my project

    i open a project updated from svn, unity shows importing small asset...after 20 minites,my project opens normally. that's not the problem.the problem is , when i close unity and restart it again,unity crashed ,showing me to send a bug.

i try to open other small project, unity opens normally. but when i open my project again,unity crashed again. then i delete all the files in Library ,let unity reimport the assets...after 20 minites,my project opens normally agin,but when i close unity,the problem arise again -- unity crashed on restartup! what can i do then? is my project to large? the Library is 643 MB. is it my project's problem?my english is not that good, i hope you can understand and help me.