Well as it takes the Tories a 7% lead to get the same number of seats its hardly fair. But you're right I think the term is malapportionment.
Actually, it's not even that. It's simply FPTP in practice.
You see, the term "it takes
whoever to get an x% lead" is actually meaningless. It's a reversal of the causality, but we like to see things more simplistically than they are, and this is effectively encouraged by the reporting and presentation of things.
Your national score in FPTP does not give you anything. It is simply a summation of your vote scores in all of the individual battles. From that, you can't really work out what happened in all of those battles. What we still tend to do, despite it being shown time and again to be only a very loose rule of thumb, is take what happened last time in all of the constituencies, see what that totalled up to be, and then estimate a changed total and guess how that would have come about from the sum of the constituency totals. And the loose rule of thumb we use is that
every constituency changes exactly the same way. Which, of course, they don't.
We sort of hope all of the differences will cancel out, or at least close enough that it'll be kind of accurate, but after each election, the landscape changes so that the projections based on the last one turn out to have been badly out.
And the projections that we often talk about are:
"How much do the Tories/Labour need for a majority?"
"What is the seat advantage that Labour (or, in the past, the Conservatives) have on equal vote share?"
"What would the lead required be to get an equal number of seats?"
So - here's a table showing what we project(-ed) for the past few elections and the next one. Notice just how much the numbers change. In the table below, the line "2005 election" shows what a projection from the last election said would be these boundaries going in. The line below is based on what actually did happen.