If you make your image 892 x 210px then it will remain that size and a copy will also be made at the lower dimensions.
If your banner size is set to large then the larger image will be used.
If you uploaded a smaller image then it is automatically resampled, resized and saved in the large and small banner dimensions. Unfortunately this can result in increased image size depending on the image content and format. I wouldn't have though a 130k image would slow down the your site loading on most broadband connections though!

I've uploaded a custom banner image at 702 x 150 resolution, the source file is 21.3kb but I noticed afterwards that viewing the web site was rather slow.  Downloading the banner from the site revealed it had grown to a massive 130kb!  Is this normal or is there a way to prevent a non required resize/re-compression of the image?

Thanks,
Richard