Because it's probably been rendered as an uncompressed file. Please press F1 then read the help files on Rendering and Compression.
I can't give you a better answer unless I know the details of your project. It's like asking "Why is this blue?" Why is what blue???
I agree with Rick's answer. Here's more detail in an entry in the FAQ list for this forum:
In general, it's nice if you read the FAQ list for a forum before posting a question: