I see nodboy has responded to this issue yet so thought I'd make a suggestion. The way that you are setting the heights looks correct, but I notice that the values you are using are relatively small. This makes me wonder if the minimum row height is being used instead. You can test this by setting something like:
_grid.defaultRowStyle.minimumSize = @(1);
I think the minimum row height might be higher than the value you are trying to set, so changing this minimum will hopefully help.