"The file is there taking up space..." vs. "my quicktime won't play movies over 20mb" (added emphasis)
What's the extension of the file (i.e. the letters that come after the period in the file name)?
If you have .avi or .mpg, then you may have a corrupt file. Especially if you have an MPEG file, then you might not have downloaded all of the packets in the right order from the host of that file. (Quite common with P2P MPEG files or MPEG files that haven't been downloaded completely.)
Does this issue persist with EVERY file over 20MB, including those that end in .mov?