that's weird – after updating to the newest version yesterday I found the selection field not compressed but too long. The 100% width is overflowing my sidebar. The edit field has the right width.
Also the wrapper ".rwmb-input" has the correct width, but the selection input inside is to wide.
I have noticed that "box-sizing: content-box" is the culprit.
It comes from
Have you also observed this behavior?
I have disabled all custom admin styles.
Edit: For now I have overwritten .rwmb-select with explicit "box-sizing: border-box"