However, if I start with the .config which workd, and "make xconfig"
and select "Supported processor types" under "Processor type and
features" and then unselect "Support Intel Pprocessors" (I have a
Ryzen) and save the config, it decreases the size of .config from 169K
to under 13K, losing almost every setting in the file.

Checking my systems, .config is 131k (AMD system, so CONFIG_CPU_SUP_INTEL is not set) and an Intel system is 116k.

Try following the same procedure using make menuconfig instead of make xconfig and see what happens.