They are on the thin side, but that comes from being weaned in winter and having to eat hay. The OP doesn't have their location posted, but considering I was mowing six acres of grass over the weekend, some of which was taller than me, I'd say they are up north where the grass hasn't begun to really grow yet, judging by those photos. So born in the fall, nursed on a cow that was probably eating mostly hay, which is lower, nutritionally, than pasture, coupled with being weaned onto hay is a recipe for thin babies. One of the reasons in the South we breed for calves to drop in the early spring so they are weaned and can eat good pasture before being taken to sale in the early fall.
The Jersey heifer will probably be fairly thin, but the Jersey bull should get pretty thick.
https://en.wikipedia.org/wiki/Jersey_cattle
As far as the Texas Longhorn crosses, they are a fairly thin and rangy breed. They will put some weight on, but they seem to do that in their second year.
