Dropped packets over multilink bundle

I am experiencing a strange behavior when connecting a multilink ppp
bundle to my ISP.

I have a bundle composed of 4-T1 links, and the number of links I have
up affect the minimum packet size I can transmit out the link. I seem
to be able to receive any packet size.

For my test, I use the ping command with a specified size.
Ie.. ping -s <x> <IP>

Below are the minimum ping size based on the number of links which are up.

4 Links: 52 bytes
3 Links: 32 bytes
2 Links: 12 bytes
1 Link : 0 bytes

Does anyone have a suggestions for what may cause this?

Thanks
-Ken