Two tamed horses, when bred with any breeding food, are producing offspring that are almost completely full sized (just looking at the horse it is not noticeable the size difference, even with an adult nearby). The pseudo-adults still behave like the babies, their legs flailing fast when they move and other behaviors, an inability to ride or saddle them, and they can be fed still to accelerate their growth to adults or allowed to develop at normal rate. In addition, when you do feed them to speed their growth, there is a very hard to notice slight size change before you finish making them adults. Once they are adults they are tamable and breedable as normal. The only glitch appears to be in the age-size they are rendered.