I think this increase is to keep the time needed to download a constant ]:) . As the average download speed goes up, so does the software size, so it still takes 15 minutes. Your old 32K program on a 300baud also took 15 minutes (I have not verified the numbers by calculator - you get the idea/point)
It is the same with mails. Now an average mail (because of XML and HTML and pictures and whatnot) takes 100Kb - but your mail store is in Gbytes. Before the mail was hundred bytes or maybe 2 kb and you only had 5Mb space. Again, the amount of mails you could store/scan is constant ]:)
I also think of the boot time of a computer. An old XT or AppleLisa took 2 or 3 minutes to boot of the floppy reading in about 150K into memory. My Win7 needs about 1.5Gb memory painfully ]:D collected from 20Gb of core files which takes about 2 to 3 minutes.
Do you see a pattern? :P
Maybe the real reason is that if one of the factors got "out of hand", like it takes 20 minutes to boot windows, then developers are forced to THINK and make it faster. But - hey - anybody can wait 2 minutes! :P And when boot goes in 1.2 seconds, then there is time to add another fancy feature because there is time.
Thus a big applicatin always takes 15 minutes to download, a machine takes 2 minutes to boot and I can only reasonably manage 20K of mails. Constancy is the result of continued change. (?!? ehr?)