What the size checker is measuring
An .xlsx or .xlsm file is a ZIP package. The number you see in File Explorer is the compressed size of that package, not a count of cells. Two workbooks with the same used grid can differ by tens of megabytes because one of them is carrying pictures, a pivot cache, or a styles part that the other never wrote. This checker opens the ZIP directory and ranks every part by compressed and uncompressed size. It does not resave the workbook and it does not delete anything.
Which parts usually dominate
Pictures and embedded objects live under xl/media and xl/embeddings. Excel often keeps the original image even after you crop it on the sheet. Pivot caches live under xl/pivotCache. The records part is a stored copy of the source rows, so a pivot built on a hundred thousand rows can outweigh the sheet that displays the summary. Shared strings hold unique text values once and point cells at them. That is efficient until the same report is pasted as values many times with tiny differences, and the string table explodes. Styles, the calculation chain, printer settings, and custom XML are smaller on a tidy file and obvious when they are not.
The repair steps live in the guide on how Excel stores the heavy parts.
How to read the result
Start with the group totals, then the largest single parts. A typical reporting file often has worksheets as the largest group. If one image is most of the package, the fix is in that image, not in the formulas. If pivot cache records are most of the package, shrinking the sheet grid will do little until the cache is cleared or the pivot is pointed at a smaller source. The share column compares each part with the file you dropped in, which is the size you actually email and store.
What a large file is not
A large file is not automatically corrupt, and a small file is not automatically fast. Volatile formulas and whole-column references can make a 2 MB workbook feel stuck, while a 40 MB workbook of pictures can calculate instantly. Use the size ranking to decide which other check is worth running. The Audit score treats size in bands, starting at 5 MB, so a modest planning model is not punished the same way as a 50 MB extract.
Limits
The checker reads .xlsx and .xlsm fully. For .xlsb it still ranks ZIP parts, which is useful, and it labels the rest of the report as partial because sheet formulas are binary. Legacy .xls files are refused with instructions to save a modern copy. The cap is 250 MB. Nothing is uploaded. The ranking is descriptive: it will not tell you which picture you are morally allowed to delete, only which part is consuming the bytes.