The menu commands 'Scale to Frame size' and 'Set to frame size' must win the award for the most badly named commands in Premiere Pro. I teach PrPro and they are almost guaranteed to generate a look of quizzical incredulity on any new users face when I tell them that the command with 'Scale' in its name doesn't adjust the scale parameter, but the one without 'Scale' in the name does...
It's time to grab the bull by the horns and fix this.
In addition...
Frame sizes ain't what they used to be. 16x9 isn't the one and only. We have social media: Square formats, portrait formats, whatever format you like. These commands need flexibility for different formats.
My suggestion:
These commands are grouped under a 'Transform' sub menu as follows:
'Set to frame size' is broken down into three components, named as follows:
Scale to Fit Frame
Scale to Frame Width
Scale to Frame Height
and, as a fourth sub menu entry...
'Scale to frame size' becomes 'Resample to current size'
...and this works in a slightly different way to the current version: When 'Resample to current size' is selected, the content maintains its current visual size, but is internally resampled to 100% scale - with 100% showing is the Effects Controls > Motion > Scale property.