What is the difference between a ED28 and ED37 base bulb for HID lights?

Answer 1

The question is wrong.

It is ED28 or ED37 bulb not the base.

ED28 bulb is smaller bulb, about 7 or 7 1/2 inches tall.

ED37 is a huge bulb about 12 inches tall. Make sure you order the correct bulb because ED28 will go in the fixture designed for ED37 but reverse is not true. It is just the matter of size but other things too must be considered like if the fixture is enclosed etc. because bulb requires high temperature surroundings.

Then bulbs come in different bases, most popular are E39 and EX39. EX39 is extended and MAY not fit in E30 base socket. There is a trick once can break the edges so bulb fit.