SQL Server DB table data size query result not increasing if data count increasing
SQL Server stores data on 8kb pages. You can't have a table that's smaller than than two pages. So when you add one row to the table, you have a minimum space consumption of 2 * 8kb = 16kb. But those pages aren't full, which means adding 15 more rows doesn't increase table size by (15 * initial size), it just means that the data page holding the initial row had enough room for more data. If you add 1000 more rows, I bet your size will increase, but this will vary depending on what's in a row and how many rows can fit on an 8kb page.

I don't know that it matters, really, how much data is in a table if you want to only look at the space on a page that's used, and ignore the space on a page that isn't used. You're taking 8 kb on disk and 8 kb in memory whether the page has 10 bytes of data or 7,997 bytes. So you will see that most space calculation scripts just take (number of pages) * 8,192 because the unused space on a page is very rarely relevant.