I recently went through an extended search trying to find a seat bag that was not too wide at the seatpost. In part the issue is the overall size of the seat bag, since it's easier to find the right shape bag if it's smaller overall.

For large bags, Bontrager and Specialized taper pretty well at the seatpost, but I found the zipper configuration to be inconvenient for accessing things in the bag.

Topeak's medium size seat bag is another to look at, if the overall volume is sufficient. Lezyne makes a very narrow bag, but their "large" bag is not really very large.

I also found one at a small bike shop on Long Island that is a brand I'm not familiar with. It's large capacity but not too wide at the seatpost. I'll check the brand when I get home later.