To do any comparison make sure key frame rate matches as it can have an impact on file size.
Otherwise what you are saying is technically difficult in "normal" circumstances. The data rate closely defines the file size. What is likely happening is that you think you are setting a data rate but you may not be.
In QuickTime Pro make sure you are setting it to a codec which has a controllable data rate such as H.264. DV, AIC, ProRes, for example, have date rates defined by the codec. Once you have selected H.264, in the Date Rate window make absolutely sure you have Restrict to (enter data rate) checked and NOT Automatic. As noted above, use the same key frame rate for both. Also make sure you have Frame Reordering checked as this uses B frames and is more efficient.
Just about the only way the same file source can result in a larger file at 400kbps than 1500kbps is if the data rate is ignored. There certainly are some other ways but I don't want to go into arcane rare circumstances.
The best way for us to confirm that there's not something actually awry with your system is for you to provide screenshots of both Compression Settings.