What sports, physical activities, and exercises are the most likely to
cause an adult's feet to become wider?  At present I am only aware of
running, jogging, and carrying an unborn child.

Hello and thank you for your question.

     Of the various foot ailments that are often seen in sports
medicine, the one most directly associated with widening of the foot
is a partial or complete collapse of the arch, called “acquired pes
planus” (acquired flat foot/fallen arch) caused usually by excessive
pronation.  Normal pronation (turning inward) of the foot is necessary
as the foot adapts to the ground. With excessive pronation, however,
the arch flattens, collapses, and soft tissues stretch. At first,
excess pronation may cause fatigue. As the problem gets worse, strain
on the muscles, tendons, and ligaments of the foot and lower leg can
cause permanent problems and deformities.
“What is Pronation”
http://www.walkingclinic.com.au/html/pronation.html

     Any sport or other activity that causes excessive pronation can
cause this condition.  The sports activities most likely to cause
these problems are running, cycling, stair climbing, and walking. 
Most of these sorts of injuries are not the result of sudden
catastrophes but occur because of what is popularly called 'overuse'.
This is one way of saying that a key part of an athlete's body – in
this case the foot -- simply can't stand up to the regular pressures
of training and competition without breaking down.
“Sports Injury Bulletin”
http://www.sportsinjurybulletin.com/archive/0123.htm

     Overuse is not limited to sports, of course.  Anyone whose job
requires him or her to spend a lot of time on foot (mail carrier,
restaurant or factory worker, farmer, etc.) risks similar problems.
“What is Ergonomics”
http://www.ehs.ucsb.edu/programs/iipp/iippresources/ergoguidelines.pdf

     So be good to those feet!


Additional Links:

Here’s an interesting article that describes acquired pes planus along
with other common foot problems
“Foot Problems in Middle-Aged Patients: Keeping Active People Up to
Speed”
http://www.physsportsmed.com/issues/1998/05may/stanish.htm

And here’s a surgical procedure to correct the condition (caution—not
for the squeamish)
“Medial Calcaneal Sliding Osteotomy”
http://www.medmedia.com/05/52891.htm


Google Search Terms Used:
   sports foot width
   "Acquired Pes Planus" wide
   sports causes of excess pronation
   
Thanks again for your question!