Why is wx.NO_BORDER changing panels within a frame

[MeghansUncle2]

In the code below (within wx.Frame.__init__) when I add "style=wx.NO_BORDER" the panels shrink to almost nothing.
Please run with and without this style to see the difference. Can anyone explain this behavior? I am learning Python and this behavior seems to me to be in error. I would like to understand why this is happening so that I can prevent it.

import wx

########################################################################
class RandomPanel(wx.Panel):
    """"""

    #----------------------------------------------------------------------
    def __init__(self, parent, color):
        """Constructor"""
        wx.Panel.__init__(self, parent)
        self.SetBackgroundColour(color)



########################################################################
class MainPanel(wx.Panel):
    """"""

    #----------------------------------------------------------------------
    def __init__(self, parent):
        """Constructor"""
        wx.Panel.__init__(self, parent)

        topSplitter = wx.SplitterWindow(self)
        hSplitter = wx.SplitterWindow(topSplitter)

        panelOne = RandomPanel(hSplitter, "blue")
        panelTwo = RandomPanel(hSplitter, "red")
        hSplitter.SplitVertically(panelOne, panelTwo)
        hSplitter.SetSashGravity(0.7)

        panelThree = RandomPanel(topSplitter, "green")
        topSplitter.SplitHorizontally(hSplitter, panelThree)
        topSplitter.SetSashGravity(0.5)

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(topSplitter, 1, wx.EXPAND)
        self.SetSizer(sizer)

########################################################################
class MainFrame(wx.Frame):
    """"""

    #----------------------------------------------------------------------
    def __init__(self):
        """Constructor"""
        wx.Frame.__init__(self, None, title="",
                  size=(800,600), style=wx.NO_BORDER)
        panel = MainPanel(self)
        self.Show()

#----------------------------------------------------------------------
if __name__ == "__main__":
    app = wx.App(False)
    frame = MainFrame()
    app.MainLoop()