I think he ripped it like that simply because he was startled from the sudden demand and blue magic makes you strong and honestly Gaster could have manipulated the movement anyways. I mean, sure, the thought crossed my mind that it would be more difficult, but not to much more. I tried both ways and when pulling quickly and suddenly like Sans did, the paper is torn fairly effortlessly and unevenly. So, in my opinion, no plot holes to that matter. In tackling the whole science between Gaster having them be different, mistakes could have been made, for one. Two, no one had ever done this before, and there was no way for him to know how either of them would emerge. Thirdly, if this is anything like birth, there is never any guarantee of what the child will look like or what traits it will have, other than the standard likeness to the parent or, in this case, creator. As we’ve seen, Sans already has similarities to Gaster. Therefore, I conclude, that with Sans being smaller and having some difficulties as not being a plot hole. If you still don’t believe me, look at it this way: if two parents have a kid who is autistic, but neither of them had autism, was the kid made incorrectly? In no means were they. In my mind, the same applies to Sans and Papyrus, in a way. You don’t have to agree with me, but I hope this possibly helped sway your reasoning! ^_^