You can't go blindly changing MCBs for larger ones. The MCB may be of that rating for a reason. Not just cable calcs.
The MCB could be supplying another cable of a smaller size somewhere. Or the cable has been joined somewhere with a smaller cable. Or the electrician only had a 16 amp MCB. Who can say? The possibilities are endless. Never assume anything without checking.
I'm sure if this had been the case it would have been mentioned - extra conductors in the MCB / joints in the cable