/var/log/wtmp is what makes the 'last' command work. It used to be there in the previous version of the OS, so 'last' worked (you could see when last you rebooted, when it crashed, when you logged in and so on). Without 'wtmp' that info is gone, and it was removed in the last OS upgrade, but on a mobile device it makes some kind of sense to remove it. Maybe. Anyway, for some reason it was updated way too much in the previous OS version so it grew fast. On the other hand it's on the compressed internal file system so it didn't take much space, those 60MB you saw would probably be just 1 or maybe 2 MB, if that.