Need clarification - are you saying she gets 20 pounds of alfalfa in the morning and then another 20 pounds in the evening? Or a total of 20 pounds in divided feeds? Either one of those is not enough, really.
I would start with increasing the hay. They key question to ask yourself is: Is she cleaning up the hay you are giving her. If she is, then she is ready and able for more. You can keep increasing the hay poundage until she is starting to not be able to clean it up. But, frankly, with a 3-month old who is undoubtedly also eating hay now, you need to technically feed for 2 horses in that pile.
When she gets to the point she can’t eat all the hay you are offering, then you should consider changing her away from the balancer and instead go to a full-blown broodmare/lactation kibble which tends to have much higher calories. To this you can add a fat formula - either flax, or oil, or a fat-kibble. I use Step 8 because that’s what we have available and it is a 20% fat kibble and they love it.
While alfalfa pellets are more condensed, they do not really satisfy the chewing urge and I have had horses turn away from pellets, but go for great gusto to the alfalfa/grass hay at much higher volumes than ever thought possible.
At the peak of lactation my mares were getting enormous amounts of hay. I use round bales, so while they were in their stalls, I was pulling the feed off the round bale and the hay pile in their stall was ginormous - easily 40 pounds and that was just their night feed. And they were eating every scrap of it. They are basically eating all night long which was just fine with me.
During the daytime, they had free choice round bale alfalfa/grass mix plus access to about 60 acres of pasture that was elbow deep alfalfa/grass mixtures. They would graze for a while, but then all would come in to chomp on the bale. On top of this, they were getting their “mashes”. The one mare I had this year who was a bit harder keeping, she got a mash twice a day. It is unbelievable how much food a broodmare can truly pack away.
Long and short, if she is cleaning up her meals, she can have more - much, much more.