Comparing an extremely casual feat from Burgess to Zoro's ult lmao.
Zoro didn't use 3 billion world because he needed the power to cut through the mountain sized Golem, a much larger mountain than the one Burgess lifted. The reason for using that attack was to lift the golem into the air and keep it in the air long enough for Zoro to locate and take out Pica's real body.
As long as Pica is connected to the ground is easy for him to escape. By forcing him into the air and keeping him in the air he has no room to run.
Zoro next attack was a non-name attack and it cut a larger part of the golem vertically.
There's your proof. If he would have needed 3 billion world then a non-name attack wouldn't have cut a larger area. That is further supported when we see Zoro used a weaker version of that attack against full body haki Pica who acknowledged he's at his strongest in that form.
IF you want to compare casual attacks we can look at Zoro casually cutting through a ice and stone/rock mountain similar in size to the one Burgess lifted.
Doesn't matter how you look at it he loses and weaker than Dressrosa Zoro.