It is a PNG file. PNG, unlike JPG and GIF(or well like gif can be), is not a lossy format. This results in larger files. In other words PNG will almost always be larger than JPG or GIF.
Furthermore, Transparent PNG's are just asking for it. Transparency really ups file size in both GIF's and PNG's. And PNG transparency is just plain better than GIF's. I don't know if it is less efficient(as in it creates bigger files) but I do know that a large transparent PNG can easily break 100kb, and breaking 200 isn't impossible.
Just for comparison sakes here is what it looks like as a GIF(I did nothing other than convert the image). Top is png bottom is highest quality gif I could save as.
As you can see PNG is the superior format for transparency when size is of no matter. GIF though is superior in compression alogrithims. Personally I use PNG or another lossless format if I feel like it for just about anything. I just don't like lossy formats. Well I suppose you could use a png-8. But it looks a lot like a GIF file would; just better compressed and lossless(iirc).
Note I am not a GIF master so it maybe able to be tweaked to look a bit better. But gifs are just a headache with any sort of complex transparency.