There are two possible answers to this.OK I will accept that as true but does Mac ever take an offensive posture? Even when he has the advantage.....in numbers or postion or elsewise...
1) Yes he did.
2) No, he somehow defended his way to within ten miles of the Confederate capital and then defended his way to Warrenton post-Antietam. And all the attacking at the bloodiest day of American combat in the Civil War must have been someone else.
In fact a good example of McClellan taking an offensive posture is day one of the Seven Days, Oak Grove. This is a bite-and-hold operation launched in the first clear weather since about the 5th of June, and it captures high ground which allows for artillery bombardment of the Richmond forts. McClellan is making it a battle of posts just as Lee said he would; what causes the subsequent disaster is that to make his offensive McClellan has had to strip his lines north of the river, as he's got insufficient men.