Came up in class, exact rule escaped me:
“much” + noncount noun,
“more” + count noun, right?
So what is:
“Much more” as in “I had much more than he had.”
and
“So much more” as in “I had so much more than him.”
Maybe the resident grammar experts will have more to add, but I believe “more” can be followed either by a count noun (“I want more jelly beans”) or a noncount noun (“I want more oatmeal”).
“Much more” would always be followed by a noncount noun (“He got much more oatmeal than I got”); the count noun equivalent would be “many more” (“He got many more jelly beans than I did”).
“Much” for mass nouns contrasts with “many” for count nouns. “The storm washed much seaweed and many dead fish up onto our beach.”
Both take more for a comparative, and most for a superlative. “That was more seaweed, and more fish, than we’d seen since the Nor’easter of 1982. That storm was the most severe we’d ever seen, and this one came a close second.”
And Thudlow makes a related point on “much/many more” 
Cool, seems obvious now, thanks. 