There is indeed a requirement to increase the frontier corps and the govt is over looking an increase ( from 80,000 to 160-240000) however the armed forces training is separate from the training of paramils like the FC or rangers and reserves are only brought in fully in times of extreme war.
As for increasing the army from 550000 to what you suggest about 750-800000 is a very expensive endeavor as we will have to house them, create and train officers for them, the pays of officers and soldiers, the construction of bases, the trainings exercises will increase as well not to mention the equipping of soldiers.
In the meantime our economy cannot even afford to make FATA a province so an increase of such a magnitude is a huge expense which quite frankly could be better utilized at other points. Rather than increasing the armed forces, how about we use that money to buy better defensive capabilities like SAM systems or high generations aircrafts which will be extremely devastating and effective.
The thing is that pakistan is a defensive army and the defensive army fighting on it territory has the advantage especially if it has amazing deterrence ( not just nuclear but extreme defensive capability like a missile defense system in the lines of S-400). You basically need boots on the ground when you are invading or are fighting on more fronts than one or are a fighting a long war. In Pakistan's strategic thinking all three are not part of it although point two is slowly is becoming a reality so we need FC corps and better defensive structure over there plus we have nuclear bombs which may bring MAD with India but with Afghanistan it will not create retaliation if say they thrust further in.
Create such defensive capabilities where the enemy is forced to think " where more is less and less is useless"
Long story short. Increase FC and get defensive capabilities. Very much better.