New Goldfish chasing the other one?

So, I bought this new goldfish today. But it has been chasing this other one , but only that specific one. It has been chasing it , mostly sticking its face in the middle of the triangular shaped tail or push the other one. It is important to note that th pushing has became really often when I fed them. If it is going to matter , the new goldfish is currently the largest in the tank whereas the one being chased was the biggest and now the second biggest , about 3/4 of the new ones size (3/5.5 with the new ones tail). Why is this happening? I thought goldfish were friendly and un-aggresive. I also do find it funny that the new one doesn't attack the smallest ones which are some kind of called hazelnut-goldfish which are a little bigger than than a hazelnut. I checked google a bit and they said it might be cause of mating , but I don't see any pink opening or whatever on the one being chased (funny cause I always umed it as a guy). So any suggestions? and why?