WAV vs MP3 file size: why lossy is smaller
How uncompressed PCM size is set by sample rate, bit depth and channels, and why an MP3 of the same audio can be ten times smaller.
The PCM formula in plain terms
An uncompressed WAV stores every sample exactly as captured, with no cleverness. Its bitrate is simply sample rate multiplied by bit depth multiplied by channel count. At CD settings of 44100 Hz, 16 bits and 2 channels, that is 1,411,200 bits per second. Because the rate is constant, the file size is just that bitrate times the length in seconds, divided by 8 to convert bits to bytes. There is no guesswork; the size is fixed the moment you choose the settings.
What each setting costs you
Every dial trades size for fidelity in a straight line. Moving from 44100 to 96000 Hz roughly doubles the file. Jumping from 16-bit to 24-bit adds another 50 percent. Going from stereo to a 5.1 surround layout triples the channel data. Because these multiply together, a high-resolution 96000 Hz, 24-bit, 6-channel master can be many times larger than a CD-quality stereo file of the same length, which is why raw session folders balloon so quickly.
Why MP3 and AAC are so much smaller
Lossy codecs do not store every sample. They model what the ear can and cannot hear, then throw away detail that is masked by louder sounds nearby, encoding the rest at a chosen target bitrate. A 320 kbps MP3 of a CD-quality track holds roughly a fifth of the data of the 1411 kbps WAV, and a 128 kbps stream holds about a tenth. The audio is no longer bit-for-bit identical, but for casual listening the difference is often hard to notice.
When to keep the uncompressed file
Uncompressed audio still earns its size in a few places. Editing, mixing and mastering want the full data so repeated processing does not stack up compression artifacts. Archival masters are kept as WAV or FLAC so a future re-export starts from the original. Short interface sounds and game samples are often left as PCM because decoding lag matters more than a few kilobytes. For final delivery to listeners, though, a lossy export usually wins on size.