Wix# does not participate in images handling of the standard UI. The images you specified are simply passed to the WiX as is:
I guess that WiX/MSI dialogs don't do resizing and jus fill the area. Thus most likely you will need to play with the actual sizes of the images.
<WixVariableId="WixUIBannerBmp"Value="images\dialog_banner.png"/><WixVariableId="WixUIDialogBmp"Value="images\dialog_bmp.png"/>