Pffff I'd hope that we marry people that we love so much that it transcends appearance. Also define a lot of weight?
Middle aged spread happens to most. Everyone winds up with a different body shape in their 60s then they had in their 20s. I'm looking at you - menopause.
So I think - no - weight gain should not be a reason.
HOWEVER.....If someone gains a considerable amount of weight and it's due to a complete turnabout in their personality then you could say irreconcilable differences and it's probably more to do with the reason the weight was gained to begin with.
To try and explain.... DH and I have never been particularly healthy eaters, nor have we been people who enjoy the gym. Being slim in our 20s though was easy - nights out, take-aways on the way home, fast metabolism, travelling etc etc kept us in shape - but our 30s hit us like a dumpster truck at 5am (to steal a quote from the film Tully).
DH and I have 'gained weight together', in the sense that the way our life has changed, and the way we've aged, and working from home, and not partying like we did in our 20s.....the weight has come from a combination of cosy nights in post-DC and all those other factors. Our body shapes have evolved with our relationship, but our relationship is not too dissimilar to the one we always had (we always enjoyed a take-away - we just burnt it off quicker before). I'd be pretty peed off if DH turned round and said- well you're not the size 8 I fell in love with.
But then I look to one of my siblings who has always been super into fitness (since a young child), leads a very active lifestyle and their spouse is exactly the same. I would say in their case - for one of them to start gaining huge amounts of weight there would have to be such a significant change in the dynamic of their relationship that it would be fair enough for one of them to leave, because for one of them to gain that weight would be for one of them to turn their back on the life they lead together.