The same issue exists in Unity - the only solution is to setup batch mode using something like Gimp to mass-resize all of the images.
However as others say, be careful as you can easily create artifacts or errors, and you need to ensure you still have access to the originals somehow.
In an optimal world you would have a completely custom art pipeline where all textures are designed to match the final image quality/optimisation requirement, but when using asset store assets this isn’t the case so build sizes can really blow up.