MDI Form - child form sizing

I have a small application which is comprised of several forms. I decided to add a parent MDI form and show my existing forms within this, for purely aesthetic reasons.
However when these forms open up within the MDI the sizing seems to have gone mad, as have their startup positions.
Can anybody tell me what basic error I am making here.
Many thanks

You're not making an error. With MDI, windows takes care of sizing and positioning in the best way for you.

If you object to this, you could probably change their sizing and position, but I suspect you don't really want to go the MDI route if this is the case.