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.

You just need to consider two things:

(i) The level of induced voltage

(ii) Mode of breakdown.

Semiconductors used in modern integrated circuits have a relatively low breakdown voltage. This breakdown also tends to leave permanent damage in the form of a 'punch through' failure (insulation layer or PN junction).

Also consider that the modern intergrated circuit contains millions of semiconductor devices but it only requires one of these to breakdown rendering the whole chip useless.

Valves, on the other hand, operate at much higher voltages. A flash over between electrodes is unlikely to produce any permanent damage as the insulation is a vacuum. The EMP may temporarily effect the operation of a circuit.

Internal structures of the valve are likely to be screened from the EMP by the outermost metal plates acting as a Faraday cage.

Following up on the search for the 1980 article by King et al in "Signal" magazine: Here is the complete citation to the King article:

Capt . Michael A . King , US Army and Paul B . Fleming, "An Overview of the Effects of Nuclear Weapons on Communications Capabilities," Signal . Vol 34, No 4, January 1980, p. 60 .

Many university libraries own "Signal" magazine. You can locate these libraries by searching in Worldcat.org. Your local public library can request a free copy of this article from any university library, using what librarians call "interlibrary loan".