As per my knowledge it is not possible we don't have the option to dynamically increase or decrease the size of the chart ....if the two values (in your case bars) have huge difference in number i.e. one value as 10 and other as 100000
it will obviously look small .
One thing you can try is to adjust the Axis scale ...Go to >chart properties > axis > and define your upper and lower limits it the expression ..and uncheck the force 0 ..this might help you you to certain extent
You have negative values in your chart, that is why the X axes position is moving up and the bars become smaller. This is the behaviour of qlikview. If you don't want to display negative numbers then try like this