Why are vacuum tubes more resistant to electromagnetic pulses than solid-state devices?
Most semiconductor devices are designed to operate at low voltage, and voltages significantly higher than their design operating voltage can destroy the components very easily. You're correct that this does have a lot to do with the size - the components inside of a modern integrated circuit are so close together that even relatively small applied voltages can damage thin oxide layers and cause semiconductor junctions to break down and conduct when they aren't supposed to. Vacuum tubes are just pieces of metal inside a glass tube. Not much there that can be damaged. And they are already operating at high voltages in most cases. Electromagnetic pulses just generate large voltages within insufficiently shielded equipment. The same induced voltages are far more likely to cause trouble in solid state electronics simply due to the nature of the components.