Frames Problem....

following problem: I have a Big 4096x4096x32 Detailmap-Texture for my Terrain. the Problem:

in 1280x1024x32 Modus have i 50 Frames and AVAILVIDMEM = 60 MB. But in Modus 800x600x32 only 15 Frames and 0 MB AVAILVIDMEM.

why ????

Playing in....

Load in VRAM=0
--------------
1280x1024x32 = 50 Frames , AVAILVIDMEM = 60 MB
1024x768x32 = 18 Frames , AVAILVIDMEM = 0 MB !! <<<<<<<
800x600x32 = 15 Frames , AVAILVIDMEM = 0 MB !! <<<<<<<

1280x1024x16 = 50 Frames , AVAILVIDMEM = 60 MB
1024x768x16 = 50 Frames , AVAILVIDMEM = 64 MB !!
800x600x16 = 50 Frames , AVAILVIDMEM = 68 MB !!


Load in VRAM=1
--------------
1280x1024x32 = 43 Frames , AVAILVIDMEM = 8 MB
1024x768x32 = 41 Frames , AVAILVIDMEM = 12 MB !!
800x600x32 = 45 Frames , AVAILVIDMEM = 16 MB !!

1280x1024x16 = 43 Frames , AVAILVIDMEM = 9 MB
1024x768x16 = 41 Frames , AVAILVIDMEM = 12 MB !!
800x600x16 = 45 Frames , AVAILVIDMEM = 18 MB !!