Just go through Brian Cloughely's book on the Pakistan Army to see for yourself how the British virtually stopped recruitment from Bengal after the 1857 Mutiny and amongst the Muslims tilted it heavily in favor of the Punjabis & Pukhtoons. I don't have the book with me right now so I can't quote the exact figures but at the end of WW2 the ratio of Bengalis to Punjabis and Pukhtoons in the Army wasn't even worthy of mentioning.
The same was the case with the Civil Services and much more so in fact because the Hindu Bengalis were more represented in the Civil Services than the Muslim Bengal and most of them left for East-Bengal at the time of the Partition. I can't seem to remember the name of the author or the name of the book either for that matter but a year or two ago I read a book written by the guy who started the Civil Services of Pakistan and he detailed, with generously well-referenced figures, of how lop-sided the Bengali representation in the Civil Services were at the time of the Partition and how ironically enough it was during Ayub Khan's ministry when Bengalis were inducted in the Civil Services and the Army on an accelerated basis by relaxing the standards for them.
And this, by the way, was happening in a developing country where a myriad other grievances and problems remained unsolved.
So try to look at it from a more panoramic view instead of in such a vacuum; so less Bengalis in the Army
baaad....baaad West-Pakistani !
There was a reason why they were so less and it wasn't because of the undying hatred of the West-Pakistani towards the Bengali !