IOS upgrade problem

I recently upgraded the IOS on my 2950, the files are zipped up in a .tar
file because it includes HTML stuff.
Because there is only 8mb flash on the switch I had to delete the old file
first then copy the new one down via tftp server, it was all going well till
I reset the switch, it did not extract the files but just copied it as the
.tar file.
Then I could do nothing with it except delete it, copy a .bin file down via
xmodem (which took nearly an hour) then start again.
This time I did it using Cisco Network Assistant and it worked fine, I
deleted the old file, copied the .tar file and Network Assistant extracted
all the files properly.
My question is obvious, why did it not extract the files when using tftp?
did I miss a command off or something?

Thanks in advance