This is touching on my previous discussion about analysis that claims Labour would have won a greater majority in 2005, and is a hard thing to write about on Twitter alone! If you're in a rush, read the next paragraph, if not then read past it. :)
tl;dr? AV means each constituency MP is representative, and where a government gets a greater majority, it is because local people have had a chance to say they don't want an opposition MP's policies before the governments. Simply put, AV ensures a more accurate picture of the policies we want than FPTP, it's less that AV would have increased the majorities, more than FPTP decreased them!
There is a concern out there that AV might entrench government majorities (which, ironically, is the complete opposite of those that claim that AV will cause coalitions...couldn't make it up). My stance remains the same regardless of people talking about greater majorities or more coalitions... AV as a system alone isn't the cause of that, it's to do with real public opinion and how it's spread nationally.
But let's just imagine we are in that 2005 scenario, or 1997 (as opposed to 1992 were it'd reduce the majority, or 1983 where it'd be more proportional but exactly the same majority!). Is it unfair? Is it wrong?
I don't think so. We are working under AV or FPTP in single member constituencies. We make our government not through national public opinion, but by representation by MPs. When our parliament is formed, each MP represents the collective opinion of their local area. Then, based on what party the MPs are a member to, we make our government.
It's important we understand, while working under single member constituencies we CANNOT form a government that is assured to be proportional. All we can do is ensure that each 1/650th (or less if boundary changes come in) is representative of our views locally.
FPTP doesn't ensure this happens. It doesn't mean FPTP is unrepresentative, just that in some cases we can't be sure, in some cases it's clear it's most likely not representative of real views. When AV is claimed to increase majorities over FPTP it is because AV is allowing the local population, who's vote must be fairly split for this to happen, to say that they would prefer the policies of the government over that of the opposition.
Whereas under FPTP an opposition MP may get a seat while over 50% of the voters oppose their policies, creating a fake extra opposition MP, under AV those same voters can rally, and rather than end up being supported by a right winger that doesn't represent their generally left wing views, or a left winger who doesn't support their generally right wing views, they end up with a candidate actually furthering their wishes for national direction.
This is why, whether AV returns a more hung parliament, or a greater majority (as it can do both, remember!), in both cases it is more representative of the nations actual desires for policy on a constituency by constituency basis, and so returns a more representative parliament, and a more representative ratio of government to opposition.