The important thing to know is that textures are different from simple images.
They are intended for use in DirectX games and as such need to conform to certain guidelines. The main thing concerning you here is that textures must always have dimensions that are a power of 2.
Because DirectX only accepts textures in valid sizes DXTBmp will automatically scale an image to a valid size.
Power of 2 means that width and height must be 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048...
So your 1024x768 image is not a valid size. 1024x512 would be ok as would 1024x1024. When you save a 1024x768 image as dds it will be scaled up to 1024x1024. Similarly if you saved a 200x100 image it would be resized to 256x128 as the nearest valid size

If the original image you started with was really dds and was really 1024x768 it wasn`t a valid texture to start with. While it is possible to create dds files in any size you like only valid sizes will work correctly in most games. If you are working with a game that does use dds in non-standard sizes (for example for interface screens or other non-rendered bits) then DXTBmp is not the means to edit them I`m afraid