Ok.. since you aren't providing specific details... and those are required for someone to help point you in some direction....
Which model are you trying to download? Have you attempted to download from their website again to see if you get more data than the first attempt? In fact, provide a link to the specific page you are downloading from just in case they have several similarly named model files. Download the file to a folder on your computer like your Downloads folder or other of your choosing, do not download to the SD card directly.
Have you tried to download with a different PC?
Please note that if the file on the download server happens to be at fault and is corrupted... We can't do anything about that and you'd have to contact Spectrum to see if they can correct it. I know the one I downloaded the other day appears to be fine but it was also bigger than what you mentioned.
So step one.. make sure the file downloaded is complete. If it isn't complete, the problem is either on the server side or something on your computer blocking or stopping the download before it has completed. If the problem is something with your computer, things get more complicated since there can be several causes.
Step two would have you check that your SD card is in good shape by having Windows test it for errors and copying a known good file like a small picture or other file that you can open to ensure it's ok.