Don't kick him in the a$$ just because he is a little small and feminine... they do grow. Plus he was only 13 months and then you put him to work so his energy was going to breeding and not growing.
As has been stated, all he needed to do was get them bred, and put small calves in them so they can slide them right out. He is worth his weight in gold if you don't get a huge calf that causes you to lose both the calf and one heifer trying to have it.
We have 2 bulls that we bought as heifer bulls... one is 10 the other is 9. We still use them on heifers and they are at full mature size now. Both are built a little smaller than some of our "cow bulls" but weigh 18 or more now. They are both gentle easy breeders.... and I wouldn't trade them for anything even if their calves STILL average 60-70 lbs at best.
We don't keep many of the heifers out of these 1st calf heifers because you don't know how they are going to do so we treat most all these as terminal calves to be sold. Have kept a couple though that look and do good. But their whole purpose remains to get the heifers bred with a calf sized so that she can have it with no trouble.
We often use the bull on them as 2nd calf animals too... especially the smaller ones, and then by the time they are pushing 4 and have their 2nd calf on the ground, they are grown and can handle any bull's calves that we have.
These might be really nice heifers... so look at the calves as terminal/sell calves that will pay you back for the time you have had to feed them.....and then you can use a "better bull" on them for the next calf and know that these girls will do a better job, and their bodies are primed to have bigger calves. Plus, any that don't milk good or mother good, can be sold and you can buy a better female to replace her if needed.
This bull has provided the step up for these heifers.