What determines the length of the vertical thumb (if that's the right word)?

Some sheets I have will move from the top of the page down to the bottom of
the data by sliding the thumb all the way down, or clicking below it.. Others
will go from the top of the page down to row 500+ when I only have data down
to row 64.

I would like to have the thumb move the window down only to the bottom of
the data.