Wow, thanks guys for mentioning me as some kind of expert in biomes I'm not, but let me try to help.
You want less, but bigger mountains, so first increase your plains:
try maybe 80 as a value. This will be all areas without elevation. Everything else will be a mountain. Then to change the mountains you already figured out how:
step_size is always multiple of 5 (else the game crash, unless you also mod the .lua files with all the math for the terrains, then it is even possible to make smooth climbs), and it is the vertical size in blocks of each layer of a mountain.
step_count is the maximum possible steps a mountain will have. With a lot of flat/plain areas, there will be not much space for mountains and they will hardly reach the maximum size, cause they need a large base. (I can't remember if there is a limit to this value)
With 80% plains and 7 steps, you will get mostly 4 steps tall mountains. Less plain allow more room for the mountains to grow more steps.
By the way, those are just the rock part of the mountain, not the grass one which can be tuned at the foothills code.
You can tune how many foothills you will get before reaching the rocks. I think temperate has 3 layers of foothill each with 10 block high and desert has only 2 layers with 5 blocks tall.
The desert file is closer to what you want, so it is better to mod your biome based on that file. If you want grass, just change the colors (or copy the colors from temperate)