There are several possibilities: pulling too tight is definately one of them, but another could be the thread you are using. If your stitches look too skimpy - i.e. they still look like X's when there are a bunch of them instead of looking like a picture - you are probably not using the correct number of threads for the fabric you are using. On each pattern there is a suggestion for both the fabric count (x's per inch type numbers like Aida 14, etc.) and then the threads used for typical cross stitch on that fabric. If you are using a bigger fabric then you need more threads.
If you could post a picture of your current work - it would be easier to troubleshoot.
Tyvek is also the stuff that they wrap buildings with to keep out the wind - so you should be able to purchase rolls (100 sq. ft. and more) at your local building supply store. It is pretty cheap - though not free...but it sure beats sewing a bunch of envelopes together for big projects!