There are several possibilities:
- The ping target ignores pings over a certain size.
- The ping packet is fragmented somewhere along the way and the target system (or some in between firewall) doesn't answer fragmented ping requests.
You can test the latter by pinging with the "don't fragment" option set (on ubuntu: add a -M do option), but that is not failsafe since some routers don't properly return an error if they drop a too large packet.