Further research answered my question. Found my answer posted by QuickTimeKirk on a related topic:
"There are two options for quality in the screen recording set up (Medium and High). Medium uses Photo-JPEG codec and High uses H.264. Medium quality actually makes much higher file sizes."
My thanks to QuickTimeKirk. Since I haven't seen any of this extremely helpful info posted here or in a Google web search, I must assume I'm the only idiot on the block. However, let's get the word out.