Violations will only be given if you land an oversized plane in a small airport (Aircrafts larger than B737/A320 on either Class Delta or Echo airports).
Violations will not be given if you land over MLW, it would just be harder to land the plane.
To find out if your airplane is too big for the airport, just go to maps and press on the airport in question. If it says “Current Airplane Restricted” on the info tab of the airport, do not try to land at that airport or you will receive violations. Although some airplanes (such as A380 and B747) are too big for Class Charlie airports, you can still land there because there are no violations if you land oversized airplanes at Charlies, only Deltas and Echos.
The above violations only apply for Training and Expert servers. You can always land an A380 at Class Echo airports if you want - only on the Casual server :)
Hope this helps!