its the center of gravity and driving style. compare to lantis which naturally has an ideal center of gravity and better suspension setup, myvi has the basic torsion beam, softer suspension and higher center of gravity which contribute to the ' buang' feel as you define it. changing to wider tires means lower profile needed, this more or less help lowering the center of gravity. changing the suspension system to cater your driving style is the best way.
also, driving style like braking during corner (not before or prior to taking corner ) is another reason. the thicker and soft sidewall on stock tires (profile 65 and above) contribute to tires flex' thus added to 'buang' sensation.