ADC sensitivity is influenced by quite a few things, some of them pretty unintuitive (e.g., some interference from unwanted signals in the RF passband can be a good thing if it provides a source of dithering that allows weaker signals to cross bit boundaries).

Bit depth is surprisingly far down the list in overall importance. To understand why, look up "FFT processing gain." Consider that sigma-delta converters are 1-bit ADCs, yet the S-D topology is used for many applications that require wide dynamic range.