A PNG texture is decompressed before sending it to the graphics card, that means it will take up the following memory:
width * height * 4 bytes (for RGBA).
In your case, that's 16.7 Mio Bytes, or 16 MB.
This is what PVR textures are for: they can be sent to the graphics memory in their compressed form, so they take up about the same memory as the PVR file on the disk.
I'm currently working on an extensive blog entry about the topic. It's not finished, but perhaps you can get enough information out of the draft that is available here.