Can't it also be "starting" instead of igniting because people "start" fires?
Hmm, I'd also like to know if "making" could be used here. They both seem like correct options to me.
Shouldn't it be 'you are lighting a big fire!' Cause 'unawasha' is present continuous. If it was 'you light a big fire!' It would be 'washa moto kubwa!' Or maybe 'uwashe moto kubwa' if it is in future