HD Font Size Key

[RiffRaffAK]

Can anyone tell me why my font sizes on my Fire HD Plus 8 are like Goldilocks? I am currently using the Georgia font on the 8th index of the 12 index font scale. But when I select the 7th index size the font looks too small. The opposite is happening on my Samsung Galaxy Tab 7FE. I am able to use the 7th index on the larger screen. But when I increase to the 8th index font size it jumps to too large!

This 'Goldilocks' too small, just right, too large font doesn't occur on my Kindle Oasis, nor does it occur in FBReader reading the same EPUB I Sent to Kindle.

The last book I sideloaded to my Fire I converted to KFX using jhowell's Calibre plugin. I set the base font size to 16.0 and the font size key to 9.0, 12.0, 13.5, 17.0, 20.0 without any effect. Adjusting the font size was the same for all my books - purchased, samples, Sent to Kindles, and sideloads.

Has any other reader experienced this, either on the Fire tablet or using Kindle for Android?