owned multiple mamiya's 330, 220, 645, all were wonderful, low maintenance camera's....
regarding weight... in comparison to what? if compared to a hassy, i take the mamiya anytime. if they are the same weight, (they're not), the low maintenance cost, reliability and versatility more than made up for it.
I would suggest trying another copy then. I may not be the most demanding person in the world, but 4x for a 6x6 negative is an extremely low bar. I wouldn't hesitate printing up to 20x20" with my copy at f/4.5. The edges won't be great, but the center certainly won't look like your description above.