TFB TV (YouTube Channel) has a couple of terrific videos comparing 9mm and 380 pistols with regards to capacity vs size and weight. This is my modified version of their spreadsheet where I weighted capacity on a diminishing returns basis (see explanation video below). If you mouse over the upper right corner of the spreadsheets, you can open in Google Sheets.
The average gunfight involves less than 4 rounds fired. Many threats are repelled by simply brandishing a firearm. So in my weighting I favor the first 3 rounds of capacity and then give diminishing value for each additional round thereafter as follows:
1 - 1
2 - 2
3 - 3
4 - 3.98
5 - 4.94
6 - 5.88
7 - 6.8
8 - 7.7
9 - 8.58
10 - 9.43
11 - 10.23
12 - 10.98
13 - 11.68
14 - 12.33
15 - 12.93
16 - 13.48
17 - 13.98
To put the size and weight values on approximately equal footing, I multiplied the Weighted Rounds/oz by three when calculating the Weighted Composite.