8 months is too early too conquer any large tract of territory by a guerrilla force.Rebelled army men and EPR hold of many cities and town until May but had to abandoned due to lack of equipment and guerrilla trained man power.Then Mukti Bahini gone under ground for training and acquired arms.From June to September was period when Mukti Bahini organized. Intensity of attack by Mukti Bahini increased by the October. Pak force started to loose it's moral. With mounting guerrilla attacks and deteriorated economy Pakistan couldn't prolong that conflict. So that air strike on western Indian air field to convert it a Indo-Pak war with the aim of finish or be finished.It was gamble. Pak junta hoped for a Chinese and American military intervention on behalf of it and thought that if it can defeat India then insurgency in East pakistan will die out. So you can see Pakistan was desperate to end that quigmire even at the risk of loosing East Pak and surrender.So Mukti Bahini did not get any time to win a big chunk of territory.before that Pakistan surrendered to India and Mukti Bahini achieved it's objective in different way. End result is important, not the method of achieving it. I hope you will realize this simple truth.
Pakistan should thank India as otherwise there would be 90,000 dead Pakistanis at the end of the war.