Are you forgetting that your level 15 gems also needs pop, supplies, and culture to run? How much space is that? You can't compare 2 buildings without all of the information.
Once you add in roads and top up that pop/supplies/culture you are looking at close to 6 expansions or 150 squares for that extra gem factory.
now compare your 1 expansion Bellspire vs a 6 expansion gem factory.
Don't forget that your BS gets better and better when you level it without taking any more space.
The +30% longer boosts from your FS might be useful too.
edit: Almost forgot, adding another gem, 2 workshops, and 4 residences from scratch will cost how much, and take how long?
I'm not saying the BS is better, just that your simple comparison was missing a lot of information.