Well thats how it was explained to me by an engineering type person a few years back, the additional grip provided through those sticky Pirellis by the extra weight was sufficient to overcome the inertia generated by that weight in the nose. There is more to the law of physics sometimes than simply extra weight = more understeer.
If the front suspension was the same across the range then the heavier engined models would smack their noses on every speed bump. Not sure if its springs or shockers or a combination of both but there is a difference. Quite simply there has to be. For everyday use the lighter engined model is superior and is the one I would chose because most of the time you are doing normal speeds and it is easier to chuck around. For that occasional high speed stupid blast across back country roads I feel the heavier engined model is more planted and is capable of putting the power down earlier, and with more grunt hence its ability to be faster. It would not be so in a race across a town though because at the lower speeds it is less weildy, now whether thats down to suspension, tyres, chassis set up or what I am not skilled enough to comment on but having driven both types that is my opinion.
Admission, going across the Quantocks a few years back, me in a 2.4ti and a friend in a 1.9ti, at 90 mph in a wide RH sweeping bend I was sat planted and happy, the 1.9 had to back off (after it had puffed its way to catch up). Return journey, swapped cars and with me in front, same bend, the 2.4 was all over my back bumper looking for a way past while I was getting very worried at around 85 at the feel of the car which was getting light and decidedly unhappy about things.
Those are my findings, take them for what you want, what I have said is my own opinion and findings. You are of course entitled to yours.
Oh, btw if you look at the history of F1 you will note that they have actually fitted heavier engines, when the rules and regulations decreed, for instance, V10's not too long ago! Not a good comparison though as with a lighter and smaller V8 but a big tank full of fuel, the cars are now heavier however, as the chassis and handling is developed, it is anticipated that defecit will have been almost eliminated by the end of the season so, a heavier F1 car will be as quick as a lighter one. How do they do that then when the law of physics says otherwise, hmmmm