I don't know the answer either, but it may be worth looking in detail at the fonts and sizes that Word uses for the two key parts of the EQ field on your home/non-home systems respectively.
Because when I select the circle and look at the font, it is in the "Asian Text" font defined for the Normal paragraph style (which happens to be the style of the paragraph I am working with), whereas the enclosed text part of the field is in the "ordinary"
font - i.e. in your example it would be in Calibri). Because the circle size may vary from font to font, this could be something that works better when a specific pair of Asian/non-Asian fonts is chosen for the paragraph style.
Other than that, what I see Word doing here is approximately as follows:
If you choose 'Shrink text', then
a. leave the font size of the circle (i.e. the Asian font character) as the size you see in the ribbon (i.e. 11 in this case)
b. do not subscript/superscript the circle
c. shrink the enclosed text to around 7/10 the font size
d. superscript the enclosed text
If you choose 'Enlarge symbol', then
a. leave the font size of the enclosed text as the size you see in the ribbon (i.e. 11 in this case)
b. do not subscript/superscript the enclosed text
c. expand the circle to around 3/2 the font size.
d. subscript the circle.
Whether Word changes that algorithm depending on other factors/settings, I do not know.