Why Sentinel-2 bands have different spatial resolution

I can provide three elements of the design of Sentinel-2 spectral band resolutions :

• there is a data volume/ Data transmission rate issue : the data volume gathered by the Sentinel-2 is huge : 1 TB per day for each Sentinel-2. I guess the mission designers decided to provide the most frequently used bands at 10m, and the other bands at larger resolution. For instance, 3 bands are only needed for atmospheric correction, which can be processed at a lower resolution.

• technology issues : Short Wave Infra Red detectors (SWIR, above 1 µm) can't be as small as the Visible and Near Infra-red detectors (VNIR, <1µm). Elementary detector size is 7µm for VNIR and 13 µm for SWIR. IN fact, the ratio (detector size/wavelength) stays roughly constant.

• Signal to noise ratio issues. The 10 m bands are broad bands, with a lot of light coming in. So they can provide a good signal to noise ratio, even at 10m resolution. But bands 5,6,7,8A are here to catch thinner spectral features, related to the vegetation "red edge". Their bandwidths are much thinner than the 10m bands, so detectors are receiving less light, which is compensated by adding the measurements of 2*2 elementary detectors.

Each wavelength has a different reflection/radiation strength. Maybe this article about radiometry helps you or the book 'Remote Sensing and Image Interpretation' from Lillesand, Kiefer, Chipman link to amazon.com