weight of empty fields

Have it a sense to store various data in the same fields to save
database weight?
I mean, if I have more record types (3 types), with half data that are
the same, but another half depending from a value of a previous field.
For clarity the second half part of data should have differnt fields for
each of the three record type, but the most can be stored in the same
filds.
For example, in record type 1 the field f1 can contain a birth place, in
record type 2 the field f1 contain phone number, ecc...
I hope yuo understand wath I mean, I'm asking if I have a real benefit
to compact data and reduce empty fields (database wight, network access
is important becasue my solution will be used over a 2000/500 Kbit ADSL)
.