Why Is BAK Different Size Than MDF?

Why are the BAK files a tad bit smaller than the MDF file?
Why are the BAKLDF files a lot smaller than the LDF file? Am I doing something wrong in this code?


use master
go

declare @DBname varchar(255)
declare @MDFpath varchar(255)
declare @LDFpath varchar(255)

Declare DBNames cursor for
select name from sysdatabases

open dbnames

fetch next from dbnames
into @DBname

While @@fetch_status = 0
Begin

set @MDFpath = '\backupserverext-hddlibrarySQL_DB' + @DBname + '.MDF.bak'
set @LDFpath = '\backupserverext-hddlibrarySQL_DB' + @DBname + '.LDF.bak'

backup database @DBname
to disk = @MDFpath
with noformat,init,skip,name = @DBname;

backup log @DBname
to disk = @LDFpath;

fetch next from dbnames
into @DBname

end


close DBNames
deallocate DBNames