Although the shape of the Earth's magnetosphere changes depending on the strength of the solar wind, I have not heard that there is an overall expansion to this field over millions of years. In fact, because the magnetic field of the Earth increases and decreases with roughly a 250,000 year period, it does undergo expansion and contraction every 1/4 million years. Currently, the strength of the field is decreasing so that the magnetosphere is also decreasing in strength and in size.
