What is the reasoning behind the smaller radius vice presidential TFR?

Source:
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AOPA ePilot Volume 5, Issue 43 October 24, 2003
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PRESIDENTIAL TFR FORECAST
Be careful if you'll be flying in Texas next week. Except for a
visit to Dallas next Wednesday, October 29, and a trip to San
Antonio on Thursday, October 30, President Bush is expected to be
at his ranch in Crawford from October 29 through November 3. This
schedule is based on AOPA's best information at this time, and
could change. Most temporary flight restrictions (TFRs) for
presidential travel have been 60 nm in diameter; the vice
president gets a smaller radius. Check AOPA Online
( http://www.aopa.org/whatsnew/notams.html ) for the latest on
these restrictions.

Is the VP's TFR radius smaller because the intercepter aircraft
employed in policing that TFR are faster? What prevents the
presidential TFR from being smaller?

Inquiring minds want to know the logic behind the difference in size
between the presidential and vice presidential TFRs.