Flavors can fade or change over time, but I doubt there would be much difference over a week or two. Typically people worry about hops aroma fading over time. I doubt spices would change much except after many months. I agree with duboman, I don't see how the size of the bottles would affect this.
Here are a couple other ideas:
- Spices added at bottling were not evenly mixed
- Temperature of the beer can affect flavor (generally colder means less flavor).
- Pouring into a glass gets more flavor (really more aroma, which is closely related) than drinking from a bottle.
- Possibly an infection is slowly eating away your residual sugars (this would probably be obvious with a sour taste and/or excessive foaming).