You may not see the holes, but they are "large" enough to allow etchant through. I hope the tupperware you are using will not be used for food
. As stated previously, the contrast between toner and copper is poor. There should be NO pinholes at all. Your iron may be set too hot, or the toner is not coming off smoothly. The pinholes may "connect" under the toner to spread the etchent. I would certainly reduce the time etching especially if it is heated.