MTU

on what does MTU depends?

You can set the MTU (Maximum Transmission Unit) size by changing a setting in the TCP stack of the machine at the 'end' of a connection. As the traffic traverses the routers between the source and destination the MTU setting can also be adjusted to a smaller value if, for example, one of the intervening devices can't support the MTU value you've set. The ICMP protocol is used to find out the biggest MTU size that can be used along the path

This MTU Size depends on which network topology thats in use. Ethernet will use an MTU of 1500bytes.

Microsoft have done an article which details the different sizes for different topologies here

Also check this thread.