I play on and own an SMP realm for me, my friends and a few other people that wanted to join, and I like to my own backups on my pc just in case something happens and for some reason it didn't automatically make a backup. I first started noticing the issue when the world size got to about 25 - 30 mb, and it has peen getting progressively worse the larger the world size is. I have heard from youtube videos that a major factor in this is your internet speed, and I cannot test this as I do not have access to faster internet at this time.