>>2945401A sensor consists of photosites that convert light to charge, a fixed amplifier, a variable amplifier to achieve different ISOs, and ADCs that convert that charge to numbers.
At the so-called "base" ISO, there is no variable amplification done. To make this base ISO lower (= to be able to process a bigger charge), you have to make the input range of the ADC bigger, or the fixed amplification lower. But then, at higher ISOs you'll have to apply higher variable amplification, which will increase noise and lower DR. It's also difficult to build a quality amplifier capable of 100x variance (note that most cameras use analog amplification up to ISO 1600 or so; higher sensitivities are achieved by digital scaling and often marked "Hi1", "Hi2" etc.)
tl;dr with a lower base ISO, quality at higher ISOs suffers.
Also, not every camera has ISO 100 as base; a lot of crop CMOS sensors have a base ISO of 200.
>>2945485you mean overexposed
