Size doesnt really matter on cockfighting but rather would be entirely depending on how that rooster had been trained off plus having that good body condition.I dont know on how these cock trainers do train out their cocks but its true that luck would be always take up a huge factor on determining the winner of the said fight.
Size matters because it has an advantage over smaller fowl. But in "ulutan", you should be wise in choosing your opponent. The longer the legs of the opponent, the harder it is to beat. They have a reach advantage, so size does matter. It is more even in derby because they have a range in weight, which makes the fowls more likely to be the same size.
Other factors count too. Like the bloodline of the fowl, health issues, conditioning, and the position of the gaff. If the gaffer is inexperienced, he may not get the right position for sure kill hits. Luck is a factor too, but sometimes it is dismissed because if you have a strong bloodline and well-conditioned fowl, the chance of winning is a lot higher.
What happens is that those kind of illegal events are organized regionally, so a central government has a lot of problems trying to eradicate it as the local authorities not only are not very cooperative but they also hinder their efforts, so they can never find the farms in which the animals are kept or the place where the fights are held, which is a shame as cock fights are clearly a case of animal cruelty as animals die or are maimed during those fights.
It is not hard to find farms of fighting fowl. They even have regular TV shows in our country. They announce Breeders of the Year, and they feature good farms to teach backyard breeders the proper way of taking care of their own fowls.
It is also not that hard to find illegal cock fights, because cockfights are very noisy. You can hear the shouts of gamblers from very far. If there is a good group of cockfight gamblers, you can hear them a kilometer away. You can just follow the noise and you will find them. It is just an accepted practice in some provinces though, even if it is illegal. It is a weekly activity, so if the cops really want to put a stop to it, they just have to put effort 1 day per week to catch the organizers and some gamblers.