If we are lucky it is "just" the way the save is read in. Most programs today assume you can read the whole file into memory this does not work when the memory available to the program is smaller than the size of the file read in. Since 32-bit programs can only access about 3Gb of memory that will be the limit on save size in such a circumstance. However, what with me being old and remembering when 4Kb was a huge amount of memory, there are ways to get round the problem. They "just" require programmers to do a whole lot more on the interface between the game and the save.