"Much speculation has been made about the possibility of Earthlike Moons around Giant planets in a stars habitable zone. I have to wonder if this is possible based on limited empirical evidence however. Why are the large moons in our system limited to Mercury's size. While Jupiter's satellite system masses far more than Saturn's as you would expect from its higher mass, it simply has more Mecury and Luna class moons than saturn rather than a much larger top Moon.

Does anyone have planetary formation modeling insight into this? Is this just a fluke, or is there some forces that tend to constrain a Moons upper size?"

List of largest objects excluding gas gaints:
Earth Equatorial radius: 6,378.14 km
Venus Equatorial radius: 6,051.8 km
Mars Equatorial radius: 3,397.2 km
Ganymede (Jupiter III) Equatorial radius: 2,631 km
Titan (Saturn VI) Equatorial radius: 2,575 km
Mercury Equatorial radius: 2,439.7 km
Callisto (Jupiter IV) Equatorial radius: 2,400 km
Io (Jupiter I) Equatorial radius: 1,815 km
The Moon Equatorial radius: 1,737.4 km
Europa (Jupiter II) Equatorial radius: 1,569 km
Triton (Neptune I) Equatorial radius: 1,350 km
Titania (Uranus III) Equatorial radius: 788.9 km
Compiled from:
http://www.solarviews.com/eng/homepage.htm

[Start rant]
Now first off I think you "could argue" against the idea that Uranus and Neptune being classed as Gas Gaints. In comparison to Earth's mass they aren't gaints.
Mass (kg):
Uranus: 8.686e+25
Neptune: 1.024e+26
Earth: 5.976e+24
Venus: 4.869e+24
Mars: 6.421e+23
Whereas:
Jupiter: 1.900e+27
Saturn: 5.688e+26
BTW, here is movie clip (5 megs),
The Core & magnetic field of Uranus:
http://www.solarviews.com/cap/uranus/field.htm
As I see it the main difference between the size of Earth and Uranus is the size of Uranus' atmosphere, which is due to it's distance from the Sun.
The other factor is sheer human ignorance- we can't see the core of "gas gaints", therefore we measure the "core" of terrestrial planets and state this as it's "equatorial radius" but don't do the same with "gas gaints".
"Apparently" we compound this error further by comparing the density using this same way of measuring.
But I'm not going to argue that these planets aren't gas gaints- because they "look" big and what you "see" is mostly gas.
[End of rant:]

Mark, as to your point:
It's a good observation, but I think there is a tendency for bigger moons around bigger planets. When I was thinking of Earth sized planets around a gaint planet- I was thinking of Jupiter sized objects or even including brown dwarf size.
-gb