When synchro came out they released the mission "play 100 synchros to increase extra deck limit by 1", i have the feeling they'll do the same with xyz.
Yes it's 15 in the tcg, also main deck is 40 in the tcg; not because the extra en tcg is 15 means we have to use 15 too. i think with 8 is more enough.
It will happen if for some reason people stop buying boxes that contains broken extra deck monsters.
If they increase the limit more players will be interested in Coral Dragon for example.
Extra deck size does have something to do with it.
I already have 3 copies of Coral. But I don't use him because I already have better cards like 3 FL Every, 3 Purgatrio, 3 Brioniac..
As soon as Konami increases the size I'm definitely putting this bad boy to use.
So much broken extra deck monsters I have, so small is my extra deck size.