Oval would still be used,
The reason you would use an oval has to do with getting inside the hole. You can only expand to two sides, so an oval is needed. Any tool that would fit through a circle and expand couldn't be round. It would have as many sides as determined by how many directions the force is applied from the center out. Bearing pullers can have 2, 3, 4 or more arms.
I dont think this would be used to pull a very tight bushing or bearing, but it would work on a round item.
I'm not at all sure of the use of these pliers. But I can see a few things about the tool that point us away from our best guesses so far.
1. The end is flat and isn'tt finished to the level of the inside of the flange.
2. The tool is keyed to keep alignment.
3. The tool is made to enlarge the business end.
4. The form suggests that the tool goes into something that is either open or bigger than the space it passes through.
5. If the plier was made to pull hard, I would expect that the end of the handles would be bent back.
Chilly