The expenses scandal began because the mole who caused the leak was angry about inadequate equipment for the armed forces, the Daily Telegraph said.
The newspaper says staff who worked in secrecy on MPs' receipts were guarded by servicemen on leave between tours.
They became angry when they learned of MPs' lavish claims when soldiers had inadequate equipment, with many working to buy better kit, the paper says.
Gordon Brown said he did not understand the motivation for the leak.
The unidentified mole is quoted as saying the soldiers' accounts of their poor equipment prompted him to leak the information.
Tipping point
The paper quotes the mole as saying: "It's not easy to watch footage on the television news of a coffin draped in a Union Jack and then come in to work the next day and see on your computer screen what MPs are taking for themselves."
The Ministry of Defence (MoD) said their "top priority is to provide the best equipment for our people in Afghanistan".
But the mole is quoted as saying that the stark contrast between conditions facing soldiers and the MPs' lavish claims "helped tip the balance in the decision over whether I should or should not leak the expenses data".
"When they're out in Afghanistan they're out there for Queen and country, earning £16,000 or £17,000-a-year, knowing they're going to take losses, while the MPs are sitting in Parliament on £65,000, with massive expenses, and meanwhile you've got bodies coming home." read more
