First of all I'm not a Tory....
Having got that out of the way, Cable is a politician, and so if his lips are moving....
Also an election is not far away, and the Liberals are painting themselves as having been the "..voice of restraint..." against the evil Tories as a way of separating themselves and preserving their share of the vote.
Maybe there're right. Personally I would look for more impartial information, somewhere like here:
UK Government spending ? real and as % of GDP | Economics Help
The Tories want to reduce government spending as a percentage of GDP, in order to reduce debt. But the sort of figures they are talking about is the % of GDP that the government spent 1999/2000/2001 - were we really that badly off then as a society?
All the parties admit that spending will have to be reduced, it's just how much and how quickly that they argue about.
Personally I wouldn't consider the defence budget to be any safer in the hands of Labour or the Liberals, and especially a Labour/SNP coalition!
Still, vote for who you want, but try to find some solid facts beyond politicians soundbites!!