These megasized mums have to have become more popular in the last ten years because when I was in HS there was nothing that big! Rule of thumb: if you can't pin it on your clothes and you need a strap around your neck to hold it up, it's too big!
I guess homecoming mums started more as a guy gave it to the girl he took to the homecoming dance, but I didn't notice that in HS. On homecoming game day lots of girls (and some guys - girls usually made garter mums for their boyfriends) wore mums to school and then to the game. I also think most people made mums when they were a more reasonable size lol.
I am in shock. To the best of my knowledge this does not happen in Maryland! But even though its outside my norm I can see how its a cool tradition. Plus its probably awesome to show off if you have whatever makes one the "best".
One of my sisters was telling me about this. She lived in Texas for a little bit and was telling me about this culture. Being from the North I had no idea this existed!