OP, I've been running this through my head for the last hour while doing some chores. I think I can explain.

There is much geometry involved with the questions you are asking, right down to how long your arms are and where they pivot etc. But without diving too deeply into geek territory, some of the answers have already been mentioned.

-"the closer your hands are to the steer tube...the twitchier the feel will be. Wider bars will feel smoother, narrower will give you less control"

I believe keatonandrew is referring to the WIDTH of the bar. If your arms are spread apart really wide on the handlebars, it isn't comfortable to rotate the fork as far, or as abruptly as with a very narrow grip with the hands touching either side of the stem. A Wide grip will require more arm movement to achieve the same number of degrees of rotation at the fork.

-The porteur bars usually put your hands right about level with the head tube, which causes the effect he was describing.

Compounding that, your weight would be shifted back a good bit with those more upright bars, leaving the front wheel feeling lighter."

I agree with fuzz2050 on this. When your bike had drops on it, the length of your stem and the shape of your drop bars put your hands out over the point of the wheel that contacts the road. Now with the new porteur bars, the point at which your hands contact the bars is likely in line with the head tube. This will likely result in a more direct feel, more like a straight bar on an extremely short stem. The weight shift due to a change in riding position is probably more important that you think too.

Also, remember that your grip position is different than before too, as those porteur style bars require something of a hammer-grip. This will give you less pivot out of your elbows than before, which might cause some erratic control issues until you get used to the position.

So, what can be done. I wonder if a shorter stem would put those bars back a little more where the hand contact point would be further away from the fork pivot. This would require a bit more movement to make the same fork rotation and should smooth things out. This is assuming you don`t mind the bars back farther, and that you have a short stem knocking around to try out.

my $.02