Fill rate limited?

In my App I draw a landscape that is multitextured with a grass texture and an lightmap.

my App loses frame rate as I resize my OGL window. That is fill rate limited as far as I know.

But what causes this? How can I eliminate the bottleneck, or at least lessen its effects?

Is it only the videocard that cant rasterize my polys? Or has this anything to do with AGP bus speed? Texture resolutions would affect this, as lookuptables get bigger with resolution, or are they?

Appreciate any ideas / answers

fillrate is the amount of pixels, which your gfx card can draw in a second … and if you want to have as much fps as possible you can speed up your app by doing some of this steps(only if you reached the fillrate maximum and this is your botleneck):
-sort your polys from front to back, this prevents that objects hidden behind others will be drawn and you get some extra pixels/more fps/less fillrate use per frame…
-use as less passes as needed, because they double(2passes, tripples 3passes and so on your pixelammount [in most cases] and this reduces your fps by: fps= <fps for one pass>/ passcount
-reduce bpp …
-reduce screensize…