At 25 months there is no reason to think she won't make enough milk for her own calf even if she is smaller stature. You do not want to over feed the calf or you will wind up with scours. And giving it milk not from it's own momma may make her less interested to take as good care of it. Besides, the more it goes on her udder, the more stimulation for her to produce a greater amount of milk. I would not attempt to do any supplementing until you see how the heifer does with the calf, how often the calf seems to be going to her to suck, how rounded out the calf looks after a few days.... If the heifer is friendly and you can touch and work with her, after the calf is born, and has had a few times on the udder, you could try milking her a little to make sure she is producing in all 4 quarters.... if the calf nurses and goes and lays down and seems content, then it is getting enough. If it is following her around and not seeming to get full, then it is time to worry.
Too many times people interfere with something that is perfectly fine. Give her a chance before you try to fix something that might not need fixing.
The calf on the jersey is nice. Glad she had it with no problems and all seems to be going well for you. Are you milking her now?