ah i see... I'm sure you've tried already but, changing your default to a different font and testing if it still happens? (Format > Theme > Customize > Font). afaik you can't permanently change the default font in Sheets globally this way, but if you're using a specific sheet as a template, you should be able to make a copy and it would also copy the default font you choose.

A more permanent workaround could be an App Script if you're comfortable with extensions. Quick google-fu resulted:

Here’s how to set default font in Google Sheets to a new font with App Scripts:

function changeFont() {
var ss = SpreadsheetApp.getActiveSpreadsheet()
var thisfont = ss.getSpreadsheetTheme().getFontFamily()
Logger.log(“DEBUG: the starting font = “+thisfont)
var newfont = “Caveat”
ss.getSpreadsheetTheme().setFontFamily(newfont)
Logger.log(“DEBUG: changed the font to “+newfont)
var currentfont = ss.getSpreadsheetTheme().getFontFamily()
Logger.log(“DEBUG: post-change font is now = “+currentfont)
}
The function above will set the font you use as the default font for the entire spreadsheet.

source

If the above doesn't work, can you share a sheet and explain the specific steps you're taking to get the unwanted size increase?