32 MB implies 4096x2048xtruecolor, or 4kx4kx16bit. That's likely not going to work on older mobile devices.
Generally, you should move BIG textures out of the atlases and make them power of two and compress if possible - this is largely an evaluation of the visual artifacts that are made from the compression, so it's hard to say anything. Do note that Unity just uses 16 bit as "automatic compression" for android because different graphics chips handle it differently. This sucks, but is hard to work around. For iOS there's only PVR compression or true color as real alternatives.
Generally, try to keep your atlases at 2048x2048 and everything bigger than ~500px, move out of the atlas.
