In most cases, G16 bulbs have an E26 base or an E12 base. Smaller E12 base G16 bulbs are used for decorative purposes.
Yes, there’s a minor difference between a G16 and a G16.5. A G16 LED Bulb has a diameter of 16/8th of an inch and a G16.5 LED has a diameter of 16.5/8th of an inch. Both diameters are approximately 2 inches, and the bulbs can fit in the same base.