cam and carb question

Hyd. lifters use engine oil to pump up the insides of the lifter.

Mechanical lifters are solid, no give. There a certain height and that's it.
The Hyd. lifters at start up and low RPM compress and do make the cam seem smaller until more RPM and oil are introduced under higher/greater pressure to pump up the lifter.

When the lifter pumps up, it becomes more like the solid lifter giving full lift and duration as the cam, or close to it.
The idea of the Hyd. lifter is vari the size of the cam making it like a smaller cam that grows to a bigger cam as the RPM goes up. This makes it easier to start the engine, give longer service life and quiter operation.

Because the lifter rises and falls, it gives the cam/engine a more flexable operating RPM range. A broader/larger range. Mostly in the low RPM range. Which is good since a larger cam can be installed and still have good to great low RPM speed, power, vacuum, idle and more torque.