I think I fixed it but I don't understand it. Before I started I used Ctrl+End and that selected cell Y1048576. There was nothing in the cell and nothing I could see in any cell between that row and the end of data on row 1348. I tried selecting the empty rows and the file size increased. During this manipulation some highlighting appeared in the column that seems to be causing the problem (another column had fill set for the entire column.)

I use fill to highlight columns for different things. For some reason there was some higlighting in the column I deleted (but not the entire column). I opened the original file, selected the column, then selected "no fill" for the column. I saved the file and checked the file size: down to 415K! And Ctrl+End now selects cell Y1348, 1348 being the last row that has data.

So there it is. "Fixed" but with no understanding of why. Can anyone shed light on this? I hate not understanding stuff.

Thanks to all who commented.