A smaller accordion can play almost anything a larger one can - you could be playing something weighing only 15 lbs. Warm up and stretch before gigs and practice sessions. Don't play for more than an hour without a break. Take a hot bath after a long gig. Do at least SOME light exercise three tmes a week.
Swimming is the best exercise for accordionists.
Calisthenics are good. Weight training at a gym is also helpful. If it's nothing but work all the time, the excess tension will be felt as muscle and joint pain.