Size of NUMBER field

I read somewhere that a NUMBER with unspecified precision takes up 22 bytes, but upon testing it with a table, I find this isn't so.
If I enter a 1 digit number, the row becomes 6 bytes long. If I enter a 5 digit number, the row becomes 8 bytes long. If I enter an 8 digit number, the row becomes 9 bytes long. If I enter a 12 digit number, the row becomes 11 bytes long - Etc.,
I got these by analyzing the table and select avg_row_len on a table with only
1 row, and one column, a NUMBER with no precision.

A physical model was just reviewed and the suggestion was made to change

any unspecified NUMBER fields to have length and/or precision. I guess I wondering though that if NUMBER doesn't hog 22 bytes automatically,
and there are no conflicts with the application, it seems it's not important
as I thought it was. Any thoughts?

• Dc.