It's more complicated than that. The truth is, you can be quite compromised in the lung area you have available, from respiratory diseases and scar tissue, and STILL be able to have plenty of breath to sing. How is that possible?
Because there are three parts to the kind of breath you need to power your voice.
1. The Inhale: Moving air inwards
An amount of air inhaled lower (at the bottom of the lungs) than that same amount inhaled higher in the chest will give much more effective breath to the voice. It should actually feel like you are inhaling air into your lower belly and back - all the way to the pelvic floor. Open and relax the lower abdominal wall, while at the same time feeling very tall. That should give you a great inhale.
The last two parts have to do with the exhale:
2. Breath Support: Moving air upwards
When it's time to move some air upwards (through your vocal cords), your diaphragm is told by your brain to relax. Because its rim is attached to the bottom of the ribcage and its dome is attached by a scaffolding-like network of ligaments to the sternum, etc, when the diaphragm relaxes it bows upwards, pressing air out of the lungs which sit on the diaphragm.
You need to support the diaphragm by tightening the lower abdominal and back wall so abdominal contents shift upwards and don't drag the diaphragm down by their weight. When you squeeze your butt, notice the accompanying squeeze in the lower abs.
3. Breath Control: Holding air back
Air moving upwards MUST be controlled (held back) when the vocal cords come together for speech or singing. When just the right air pressure is applied... not too little, not too much... you will not feel a tightening against that pressure in your throat, neck, jaw or shoulder muscles. It will seem as though your voice floats out of you. Breath support must occur and must be balanced by breath control. Breath control is enabled by keeping your lower ribcage wide... just like with the inhale. The difference between inhale and exhale, then, is in the expanding and contracting lower abdominal area. The ribs stay wide.
With these three parts working properly, you will sense your breath falling in... then being squeezed at the pelvic floor, not the ribcage. This low squeeze will cause you to have "laser beam" breath rather than "flashlight spread" breath.
The truth is, it doesn't take much breath to sing, if the breath is applied properly with these three parts working together in synergy. This is the "Power" part of "Power, Path & Performance". Put it together and you won't believe how long you can hold notes (just ask some people who sang in "Runaway Home" last week - the last note of the last song was a killer!)
What are your experiences with breath issues? Is there anything more specific you'd like to know or share?