I think KuKulzA28 comments on being 'adaptive' is very relevant. New colonies often meant new terrain and climatic differences.....something the locals were used to fighting in .....their martial traditions and tactics would have been 'moulded' by years of conflict.
One of the 'known' tactics employed by the colonialists would have been 'divide and rule'.....why directly fight the natives....when you could 'manipulate' tribes/factions/smaller kingdoms to fight each other.
I also believe that, naively, European invaders often under-estimated the indigenous people ....believing that they (Europeans) were superior in both technology and knowledge. Often a costly mistake ....Zulu springs to mind
...often after such defeats....the colonialists retaliation was often brutal, often increasing their army's numbers, and arms to ensure victory.....to save 'face' and 're-establish' their supremacy.
Another consideration is the fact that the indigenous people would generally fight more 'fiercly' .....afterall they were fighting for their homes, their people and their way of life. I think there is a quote somewhere which basically says something like 'one man that is fighting for a belief....is worth ten that have been paid to fight'