I have a similar problem, though the phone plays the default Sendo_Ring after a couple of seconds. Trying to work out why, I have tried several different ringtones and while I have not been able to work out what is going on, the size of the ringtone seems to be nothing to do with it.
I have a similar problem. I have several different ringtones on my phone which have been assigned to different people. But very occasionally it will revert to a default ring tone or the alarm tone if one of these contacts call me.