Spreadsheets size issue

### Description

Midsize spreadsheet size grows by large chunk (several MB) when the font or font size is changed for all cells
It is accompanied by long save time (tens of seconds) and delays to display the spreadsheet properties (tens of seconds)

### Steps to reproduce

1. import the test file attached (146 kB) in a newly created cryptpad spreadsheet - size should be around 1.33 MB
2. select all the cells and change the font size from 11 to 10 - time to save approx 15 s; file size : 11.26 MB (6 s to display the file's properties)
3. select again all the cells and change the font size back to 11 - time to save approx 16 s; file size: 21.19 MB (6 s to display the file's properties)
4. select all the cells, change font to arial and font size to 12 - time to save approx 47s; file size: 52.29 MB (10 s to display the file's properties)
5. the size and time to save continue to grow when properties are changed (various tests done with font size, background and text color)
6. the same test on onlyoffice.com (7.5.1.23) with the same file and browsers leave the file at the same size and performances after many changes of font, font size, color etc.
7. same results from firefox (v 121.0) and chrome (v 120.0.6099.131)
8. if you export the file, you get the original size of 146 kB, which reimported in cryptpad is 1.33 MB

test file
[test-size.xlsx](https://github.com/cryptpad/cryptpad/files/13849443/test-size.xlsx)

### Relevant logs

_No response_

### Expected behavior

Expected behavior would be to keep the same size and same performances regarding save time and time needed to display the file's properties. With the current behavior, one can reach quickly hundreds of MB with save times longer than the minute for a midsize spreadsheet (30 columns/800rows) - which is in effect unworkable. The only way found to shrink the file to its original size is to export it, delete it on cryptpad.fr and import it again, which is not usable in real life.

### What opreating system are you using?

Microsoft Windows

### What browsers are you seeing the problem on?

Mozilla Firefox, Google Chrome

### Extensions

Google docs offline extension (on Chrome)

### Version

5.6.0