Grow Document With Updates Or Make A Separate Collection

This might be an obvious question for some pros but I am a little bit
confused.

Lets say I am making an application lets say a "Weight Loss" app where
a user each day logs in and reports what their current weight is. It
seems there are two ways to do this one way is to just make a single
document like:

{
  _id : ObjectId("4e77bb3b8a3e000000004f7a"),
  when : Date("2011-09-19T02:10:11.3Z",
  user : "alex",
  start_date : Date("2011-10-10T02:10:11.3Z"),
  start_weight : "210",
  weight_log : [
    { weight : "209", when : Date("2011-09-20")},
    { weight : "208", when : Date("2011-09-21")},
    .....
  ]
}

Obviously as each user adds weight data to the document the document
is ever increasing which could have a very serious impact on the
actual data being used per stored document due to padding and how the
collection decided over time to pad each document. Is it ok to have
documents which vary significantly in size in the same collection. For
instance user "Bob" might only track his weight once a month, where as
user "Alex" might track his weight every day. Over the course of a
year "Bob" would have 12 weight_logs where "Alex" could have
hundreds.

The other way I was thinking about this would be to make two
collections one "users" and the other "weight log", each weight log
document would have the user name (or some other id) and the date and
weight... My concern about this approach is that this collection could
be huge depending on the number of users utilizing the system.

What is the proper or "best" MongoDB way?