The Commons defence committee questioned whether the terms on offer were "fair or appropriate" and dismissed Ministry of Defence explanations of the "shocking" difference.
Insufficient attention had been given to retraining soldiers, sailors and airmen for roles the military faced shortages in, it suggested.
Around two in five of 2,860 servicemen and women laid off late last year in the first phase of a huge reduction in manning levels were made compulsorily redundant.
Philip Hammond explains why the armed forces is still recruiting
In order to avoid distorting the structure of the armed forces, making them too top-heavy in the future, they need to go on recruiting people at the bottom even while they're thinning out the numbers further up the ranks.
In contrast, the first two tranches of redundancies in civilian staff - set to total 15,000 over several years - were all done on a voluntary basis, the committee heard.
The MoD's senior civil servant, permanent secretary Ursula Brennan, said that was partly because civil servants were more "flexible" while the armed forces tended to have "specific trades".
Defence minister Andrew Robathan pointed out in the Commons that the armed forces had been "less forthcoming" with applications for voluntary redundancy than civilian staff.
Both were condemned by the committee as inadequate explanations.
"The argument that civilians are flexibly employable whereas the military are not runs contrary to our experience of the breadth of the military training we have witnessed on operations," it said.
"The MoD should set out what opportunities and encouragement it gives to those in the armed forces who face compulsory redundancy to retrain, especially into 'pinch point' trades.
"For military redundancies to be compulsory in 40% of cases, yet for civilian redundancies to be compulsory in none, is so grotesque that it requires an exceptionally persuasive reason.
"We are not persuaded by either of the two reasons we have been given."
A second tranche of military redundancies were announced this month under the Strategic Defence and Security Review (SDSR) intended to help plug the £38bn black hole in the defence budget.
Up to 2,900 members of the Army, 1,000 members of the Royal Air Force and 300 members of the Royal Navy, including military top brass, were told they were losing their jobs.
Defence Secretary Philip Hammond said: "The select committee's report is simply wrong in what it says on military redundancies.
"Every opportunity is being given for military personnel to retrain either for alternative roles in the Armed Forces or in civilian life, but the simple fact is we have to tackle the massive deficit we inherited from Labour and the huge black hole in the defence budget."
He told Sky News the redundancies were "regrettable" but he did not believe they deserved the term "grotesquely unfair".
Asked why there continue to be recruitment adverts, Mr Hammond admitted it was "counter-intuitive".
But it is vital the armed forces continue to get more staff into the junior ranks even while they "thin out" the number of jobs at the top, he said.