Explain how gaining weight isn't the exact same thing as gaining mass...

And I disagree that it's oversimplified. It's basic bodybuilding 101 fact. A caloric deficit will cause a person to lose weight. I caloric surplus will cause a person to gain weight. Period.

If during the course of this caloric surplus, a person is actively taking part in physical activity where the muscles are breaking down and being rebuilt, and the caloric surplus is not too great, most of the weight that person puts on will be muscle. The greater the caloric surplus, the greater the chance is that the person will also gain fat.

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To the OP -- I'd just recommend riding at the level you currently are. Incorporate some type of full body workout twice a week. This can be as simple as a press (chest), a pull (back), an overhead press (shoulders), and a leg movement, twice a week. You're not going to accidentally get huge.

Also, if you're serious about results, calculate your daily caloric requirements. To gain weight eat about 300 more calories per day. To lose weight, eat about 300 calories less. 300 is the starting point at which most people will see results and can be fine tuned, normally in increments of 100 calories at a time in either direction.

PS...Only a nutritionist is 'qualified' to give nutritional advice -- I've been a certified personal trainer for over 10 years now (hobby), and even personal trainers are not 'allowed' to give nutritional advice.