Another case that has occurred recently during "capture a hex" scenario. It is easier for a light mech to push an assault than other way round. That is because light mechs substract -2 from their physical attacks TN (p.88 Tac Ops) and all it takes to push another mech is one successful piloting roll. This is a very counter-intuitive ruling- pushing requires to apply force and some of it comes from pushers weight, so a heavier mech should have an easier task here no matter if it is pushing or is pushed.