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I have this png header image with transparency that is quite large 2500x1236, but only 343KB. I don't need or want it that big, so I throw it in PS and crop it down to 1666x444 and save for web, png24 tranparency, and the new, much smaller image, comes out of Ps at 585KB. WTH? How could the file size nearly double when it should be cut nearly in half?
Is this a Ps quirk? Should I just always use a png optimizer like pngOut?
When you resize an image with only a few colors, the resulting file, whether it has larger or smaller dimensions, is likely to have more colors (due to interpolation) and therefore be less compressible. Also if you started with binary transparency the result will likely have full alpha, which is also less compressible. Finally, your "bit depth" (actually "sample depth") might have increased. To regain the high compressibility you'll need to reduce the colors, number of different alpha values, and sample depth to be like those in your original image.
