The 40 cf tank is almost 6" longer than the 30 cf. Both are 5.25" diameter. To the uninitiated, that extra 6" makes the 40 look a lot bigger and more unwieldy than a 30. It's nearly the same length as an AL 80 or an HP 100 that you would have on your back. In reality, 40 is no more difficult to clip on or swim with than a 30. The 19 is definitely a more compact tank being shorter and skinnier than even the 30.
Here's a link to the dimensions.
SCUBA | High Pressure Aluminum Gas Cylinders | Composite Cylinders
Here's a link to the dimensions.
SCUBA | High Pressure Aluminum Gas Cylinders | Composite Cylinders