I'm from a very small family, both my parents are only children, so me and my two siblings are both my grandparent's only grandchildren. DH's parents are from families of 3 and 5 children, so lots of cousins, aunts and uncles etc.
My family have always spent quite a lot on Christmas. Now we're all working we would spend ~£50 each on parents, siblings and grandparents. My parents and grandparents would probably spend about £100 on each of us. We're not like super rich or extravagant generally, I think we all just enjoy choosing and giving gifts to one another that we enjoy.
DH's husband tend to spend around £20 on one another. They're actually probably slightly better off than my side of the family (I don't know exact finances obviously, but both parents work in a well paid job) so it's a choice rather than a funds-based-necessity. I think that's a fine system.
However, DH is asking why I'm spending more money on my family than his family. I was slightly surprised as he had told me they usually spend £20 on one another so I'd stuck to that.
We haven't really come to a conclusion about it, DH thinks now we're married we should spend less on my family because we're buying for more people (though we've also got both our wages coming in). I don't think you should worry too much about how much gifts reciprocated costs and am happy to give something of a higher value than I would receive if it's something you know they'll like and use but I don't think it seems right to spend £20 on both our younger brothers (similar ages and stages) when one's likely to spend £20 on us and the other £50.
So
YABU: You should spend the same amount on members of both sides of the family regardless on what they're going to spend to you.
YANBU: Both families have a system that works for them, it doesn't matter if you're spending the same amount on each.