Rookie mistake ruined spreadsheet

In Excel, I tried to hide unused columns and rows. But I did it by selecting
the first unused column, used CTRL+SHIFT+RIGHT ARROW, right-clicked an empty
cell, went to Format Cells, clicked the Protection tab, then checked
“Hidden�. I hid the unused rows in the same manner. It worked, but my file
size went from 15K to 3200K. I tried to undo the mistake by using CTRL+Z,
doing the same technique to unhide, clearing formatting, and deleting columns
and rows on the used cells, but it remained an extra large file. I ended up
copying the valid cells to a new workbook to correct the mistake, but I am
still baffled as to what happened. Do you have any suggestions ho to remove
the unused cells from a spreadsheet, in case I make the same mistake again?
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MCP, MCDST, CompTia A+