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.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles