here's good reply on this problem from another forum:
Originally Posted by The Jack of Clubs
the infuse is older than the galaxy nexus so theres no datamedia or mtp. the 16gb storage is typically called the internal sdcard (nonremovable) because it acts more like an sdcard and doesnt share partition and storage with the data partition. the data partition is called internal storage, which on the infuse is about 2gb, though a little less on cm10.2 because the system partition was made bigger for newer versions of android. if your having low storage issues on the data partition, youre going to probably have to move some of your apps to the internal 16gb sdcard by going into settings > apps > on sd card and moving some of them over. i would not recommend moving over apps that sync or run in the background, but if you have a lot of games, those typically move fine. i would do that until you have atleast 400mb of the internal storage free. ive talked to araemo in irc how hard it would be to setup datamedia on the infuse to combine the internal sdcard and data partition. long story short, rather difficult and it would be too big a hit on performance.
to support this, here's my story: I have already went back to Froyo, and saw that internal storage is 1.8 GB, now I'm back to 4.3 ROM, and it's 1.26 again
looks like I always had it all partitioned, but I haven't noticed it until system partition got changed and internal storage became smaller.
it might not be true for all other phones, as they can be partitioned differently, but it's true for samsung infuse, and probably for a number of other samsung phones.