I've been using x264vfw + VDub to make Youtube vids. I usually set threads to "0" for my quad core. It has worked fine like that for many videos. However, very recently this causes the processing to fail on Youtube. I narrowed it down to the threads setting by uploading many small test videos (varying individual settings among them).

I noted too that the file size between encodes with a threads setting of 1 (default) have a different file size than those with a different threads setting.

I mean: I'm not expert can you explain why happens that, please?

Situation that file size of encodes differ with different number of threads is normal (supposed) behaviour. I don't know why it fail on Youtube (this is the question for them). Do you also have problems playing them locally? May be you forgot to activate "VirtualDub Hack" option?

Can you explain this please: "file size of encodes differ with different number of threads is normal (supposed) behaviour" ?

What is not clean in this phrase (my bad English or what)?