The Labour leader made the extraordinary remarks as he dismissed the need to increase the defence budget.
The veteran left-winger also risked enraging British troops by insisting investigations over abuses in Iraq and Afghanistan should continue.
Mr Corbyn was speaking at the Labour conference in Liverpool after trouncing rival Owen Smith to be re-elected to the party’s top job.
The CND member has been trying, and so far failing, to make scrapping nuclear weapons Labour’s official policy.
And he was recently ridiculed as naive for suggesting that he would install a ‘minister for peace’ if he ever wins the keys to Downing Street.
It emerged last week that MI6 is to recruit 1,000 new spies following a stark warning from its chief that the threat from Islamic State-style terrorism will last a ‘professional lifetime’.
Alex Younger – known as ‘C’ – said ISIS-style terrorism posed a ‘persistent threat’, driven by the internet revolution and the breaking down of international barriers.
In a rare public appearance at a security conference in Washington DC, he said ‘deep social economic and demographic drivers’ meant there was little sign of the ‘enduring’ danger disappearing soon.
The extra 1,000 staff would mean a 40 per cent rise by 2020, to a total of 3,500.
But asked about the expansion today, Mr Corbyn told the BBC’s Andrew Marr show: ‘I don’t necessarily think that is particularly necessary.
‘I am unclear as to why they want to be so much bigger.’
Mr Corbyn refused to condemn the controversial Ihat process for investigating alleged abuses by troops in Iraq and Afghanistan.
There has been criticism that soldiers have been hounded for years over spurious claims, with cases often funded by legal aid.
‘I have spoken to a number of soldiers that have served in Afghanistan and Iraq and I recognise the awful conditions that they were asked to serve under, and the difficulties they had with that,’ Mr Corbyn said.
‘But I do think there has to be a recognition that we have signed up for international law on the behaviour of troops.
‘America is going through the same experience, as do other European countries even though they’re not signed up to the International Criminal Court. So I think there has to be investigations. Saying never to prosecute I think would be a step too far.’
The UK has committed as a member of Nato to spend 2 per cent of its GDP on defence.
Asked if the budget should be higher or lower, Mr Corbyn said: ‘I don’t think it should be any higher.
‘I think it should be efficiently used, but I also think the defence budget should also be used where necessary so that Britain is very good at actually giving aid and comfort during emergencies. Look at what we did during the Ebola crisis and other things.’
He added: ‘We’ve been through a period of putting a lot of money and a lot of troops in a very dangerous place and we’ve lost a lot of troops as a result of that. I think we need to reflect on that.’
Mr Younger’s intervention followed a string of terrorist attacks on continental Europe over the past 12 months, including the November 2015 attack on Paris, the worst in its history, and the July attack in Nice when a lorry mowed down crowds celebrating Bastille Day.
In March 32 people were killed in twin suicide attacks in Brussels.
Mr Younger did not mention the rise in staff numbers, but did warn of the damage done by the leaking of secrets by Snowden, and said developments in technology presented both an ‘existential threat and a golden opportunity’ to security agencies.
Asked if the terror threat from groups such as IS and Al Qaeda had reached its peak, he said: ‘I would like to be optimistic about this but we have got quite long experience of this phenomena now and I see it very much as the flipside to some very deep-seated global trends, not least of all globalisation – the reduction of barriers between us.
‘It’s a function also of the information revolution and the capacity for ideas to travel. It is fuelled by a deepening sectarian divide in the Middle East and there are some deep social economic and demographic drivers to the phenomenon that we know as terrorism.
‘Allied with the emergence of state failure this means that, regrettably, this is an enduring issue which will certainly be with us, I believe, for our professional lifetime.’