A texture has a size, lets say 256*256.

Realsize means, that if you set it to 254*254 the engine will show/render the image without the left, right, top and bottom line of the picture.
This sounds weird, but when using antialiase the "blurring" of the image reads pixels next to the pixel it is rendering in all directions and calculates middle values. The pixels on the borders have no "neighbours" and so black is assumed as pixel on the concerning side. Thats why you get the black border.

This is only my understanding of Realsize. I guess Lifepower can tell you if I am right or not :lol:

EDIT: Another hint. I examined your vtdb and as I can see you use texture sizes up to 1024*1024.
These are not supported by older graphic cards. So if you want your game as compatible as possible you have to tile your textures in pieces of 256*256 at maximum. I posted a topic here related to this and other users told me that because as I used bigger textures and textures of non power of 2 (eg 800*600) they were not shown on different graphic cards.