Why a workbook balloons to hundreds of megabytes, and what actually shrinks it

A file that grows without the data growing is nearly always one of a short list of causes. In rough order of how often it is the answer.

• The used range is enormous. Something once touched row 900,000 and the file has remembered ever since. Check where Ctrl with End takes you. If it is far past your data: delete the entire block of empty rows and columns below and to the right, save, close, reopen. This alone routinely takes a file down by most of its size.
• Formatting applied to whole columns. Selecting column A and setting a fill colours a million cells. Format the used range, not the column.
• Conditional formatting rules that have been copied. Open the rules manager and look at the count. Copying rows duplicates rules, and thousands of near-identical rules is both size and slowness.
• Images and shapes pasted at full resolution, including invisible leftovers. There is a selection pane that lists every object on the sheet; people are usually surprised.
• Volatile formulas everywhere. Not size but the same symptom, since the file feels unusable. Anything that recalculates on every change makes a large sheet crawl.
• Old defined names and links to dead workbooks. Both accumulate silently.

If it is still large: the file is a zip. Rename a copy and open it, and the folders will show you where the weight is, usually one sheet or one media folder. That takes a minute and turns guessing into looking.

The structural fix: when a workbook is genuinely holding a lot of rows, move the data out and keep the workbook as the view. Spreadsheets are excellent interfaces and mediocre databases.