I find form and fitness affect how a bat feels for you, when I was regularly doing exercise, I felt I could use a slightly heavier bat and it felt it picked up the same as what I normally used. After pigging out for a few months, of course your fitness and form goes, and I felt that the bat took far too much effort to swing and felt heavier on the pick up etc etc. I think regular fitness and shadow batting everyday is pretty important, even if only for a few minutes, but remember that your muscles aren't always in the same condition they were the day before, ie. could be suffering from fatigue, which can affect the way a bat feels on that particular day.