In a Circular Gauge that contains a Header and a Label Annotation the Header TextSize is smaller than the Label FontSize. This is only for Android. With iOS and Windows 10 they are the same size. This only started to occur with version 6.4.
In the attached picture, the E and F are label annotations, and the 1/2, Fuel Level, and 90% are headers, all with a text and label size of 18. In version 6.3 and with iOS and Windows 10, they are all the same size as the E and F.