First is proper fill level and tank level on the ground not tilted. Assuming those are already ok, I think you are sucking air on the inlet.
Cavitation is when the inlet liquid turns to vapor, then back to liquid when compressed. It is dangerous to the pump so poster above on the right track but for different reasons.
I think what you are seeing is 'aeration' or sucking in air into the fluid.
Cavitation won't increase tank volume. Aeration will big time. and also damage pump.
Cold thick oil takes more inlet 'vacuum' to move into the pump at a given flow rate. That high 'vacuum' or low absolute pressure can cause cavitation, but it can also cause leaks on the inlet line, hose, ports, or shaft seal, which leads to aeration by sucking in outside air even if the leak doesn't leak oil out when parked. Suction strainers/filters make things worse due to restriction if not oversized properly.
Releasing air takes a long time, especially cold oil. Check next day and see if level is back down. Then check carefully for leaks where air can go into the suction line. Pack some grease around the pump shaft seal when starting up and see if it changes anything temporarily. i.e. takes much longer to aerate.
