A pump is a pneumatic cylinder with a one way valve. Air comes in from the outsite and goes out through the tube. A large pump does need more power to press the air out of the cylinder then a small one. The area of the piston is larger and therefor it takes more force to push. This type of pump has a spring which pushes the piston back. The construction of the compressor must be very strong and you need one or more strong motors to propel the compressor.