The solution for this situation is to relocate space between volumes with Paragon’s CampTune utility. CampTune rearranges free space between HFS+ and Windows partitions and shrinks existing volumes to free unallocated space on a hard disk, and create new partitions without the need to use Boot Camp at all.
An easy alternative is to use Paragon CampTune from the beginning: resize the primary HFS+ volume, create second partition, format it as an NTFS volume and install Windows directly.
Best tool there is, and it has other features as well.
I keep telling people, 40GB minimum with 60-80GB if you can. If you want to try Windows 8 when it comes out you can maintain 7 and rollback if it doesn't work. An SSD also needs an added 10%.
Updates, page, cache, programs, temp space, hibernation - they all eat up extra space. Too little and the next set of patches and you could find it won't login or function.
GPT and EFI along with Lion/ML Recovery plus Mac and Windows more than that is probably caused by having to work around fragmented partitions and free space. Also, Windows 7 & 8 normally set aside 100MB system reserved since 2009 which Boot Camp Assistant doesn't work with or understand - another reason.
Ofrten when you remove and recreate you loose some space, overhead.
CampTunes would as I said been 1st choice. It even can be used instead to setup format and install Windows without having to go through FAT => NTFS because Apple does not have the rights to NTFS.