The Seagulls boss saw Giroud at his best with two goals in his side's narrow 3-2 FA Cup defeat by Arsenal on Saturday, while Argentine striker Ulloa also scored on his debut.
"I knew Giroud was strong but, maybe because I'm not used to the Premier League, he was massive," said Poyet.
"He was very strong. I won't say unstoppable but for us he was not like the normal striker from the Championship. He put himself between the ball and the player and he didn't lose too many challenges.
"When you are defending against a very good player and you are not too smart and it's a very good team, it's almost impossible.
"At Championship level, Leo should give us that power, that holding and strength that we didn't have.
"I'm pleased for him. That's what's difficult for me. We lost and I don't like it but it was a good start for Leo. He scored, he was feeling the atmosphere which was incredible at that time and we'll see where that takes us."