Because Science Selective are extruded pellets, they weigh about half of what the same volume of Oxbow pellets weighs. So if you’d normally give 2 tablespoons Oxbow you should give 4 tablespoons of the SS.
The alfalfa is balanced in pellets, the amounts of Calcium and protein (and often the Ca/P ratio) are declared. It doesn’t matter if the protein and calcium comes from alfalfa or from other sources, as long as all the essential amino acids are there and the calcium content isn’t too high.
I agree with the others: Pellets are yummier than hay and if a bun gets a lot of pellets, the bun will choose pellets over hay and hay consumption will go down. And that’s bad for both teeth and tummy.
I have never fed any adult pet the full amount recommended on food packages. Half is often about right, or you’ll soon have a little fatty on your hands. My bunny Bam weighed 2 kilos and according to Science Selective he should get about 8 Fibafirst sticks per day. He was prone to putting on weight, so he had 1 to 1.5 sticks per day. My bunny Vilde gets 2-3 sticks per day because he’s a lot more active.