In a recent visit to London, there is a WW2 memorial which talks about the 2.5MM Indian soldiers who volunteered to fight in WW2 while India was under British colonial rule. It is considered the largest volunteer army in history.
However, I would say that anecdotal conversions in India imply that many of those men volunteered due to lack of food/employment at home, and some books state that there were reduced requirements so sick/anemic people were also joining. The implication is that this was basically "forced conscription" and not people volunteering on their own volition.
Is there historical data/evidence one way or another? I'm sure some people volunteered on their own accord, but can we measure how many people were essentially forced into joining through either coercion, starvation, or other poor living conditions?
It's an interesting question, one that I do not have a definite answer to. I do know, however, that whether or not they volunteered, many Indian soldiers saw fighting in World War I as a path to equal status in the eyes of Great Britain.