I think there are a couple more factors in the equation than just the size and the duty.
If your present company is stable, treats you well, pays you a fair salary on time and they want to promote your career, your colleagues are people you tend to get along with just fine, then that is worth a lot for your overall job satisfaction. It might not be the case in the other company, if they are just offering you a right hand seat in a bigger aircraft but lack in other areas.
However long-term it might be ok to sacrifice your comfort-zone, take a knock and get the experience on a heavier aircraft and then move on again if necessary.
You will have to look at all of these things to make a good choice.