It is propl. not a question of midi-size, more of midi-version. Midi music comes in different versions, and some are quite different from std-midi, especially in header. Have you tried to open the file in other midi-supporting programs? Here i would recommend Anvil-studio -free, and versatile. If the midi is flaw-less, You can resave the file to midi0, whitch lmms will open (How big is that midi? Where do you have it from? )
http://www.anvilstudio.com/