Watermarking & File Sizes

There's probably an obvious answer to this, but I'm having trouble
turning anything up with google. When I add a small watermark image to
a large PNG, the resulting file is massive.  It seems like I must be
missing an option, since there isn't that much data being added to the
large PNG.

Identify from the watermark:
logo.png PNG 125x50 125x50+0+0 DirectClass 27kb

Identify from the large image:
output_files/out.tmp.png PNG 750x1388 750x1388+0+0 PseudoClass 256c 5.9e+02kb

Identify from the composite:
output_files/out.png PNG 750x1388 750x1388+0+0 DirectClass 1.6mb

This is the composite command I'm using:
composite -dissolve 50 -gravity SouthEast -strip -quality 95 -depth 16
-colors 256 logo.png output_files/out.tmp.png output_files/out.png

I know it's not as simple as 592KB + 27KB, but 1.6MB seems extremely
large. Anything I can add or adjust in the composite command?