Ok here is the deal.  Windows will remember the size of the Explorer window of the last Explorer window size that is closed.  It will take the last Explorer window you close if you are using more than one Explorer at the same time.  Those goofy pop up windows count as well and if that is the last Explorer window you close, the next time you run explorer, it will use that size.  Usually the culprit will be this as the pop up windows are hidden behind your maximized Explorer window and you close the pop up after you exit the Explorer you were using. 

 

Now here is a trick to force the Explorer to open up maximized every time.  To do it you need to create a shortcut to the Explorer on your desktop.  This is because the Explorer icon that is on your desktop can not be set the same way as a shortcut.  Anyway, create a shortcut by right clicking on the Explorer icon on your desktop and then Create Shortcut.  Next right click on the new shortcut you just made and select Properties. In the Run: box, use the drop down list and select Maximized. Click OK and that's it.  When you use the shortcut icon to run your Explorer, it will load maximized and when you use the original Explorer icon to run your Explorer, it will act the way it does now.