Is your production server using suhosin? Suhosin is often a culprit for limiting the request sizes, both GET and POST. They have separate limits for POST and GET sizes (and in fact one affects the other from what I remember, thus both have to be increased).
Take a look at your phpinfo() and see if there's anything about suhosin there.
Suhosin values that may affect you:
(+ their equivalents for value and var length)
Also, if you're uploading files, then this too is limited by suhosin:
suhosin.upload.max_uploads (which is, consequently, 25 by default)