I think that you problem is a normal behavior of ssrs, let me explain:
If you have a main report with lets say 1 sub report in it. and your subreport is a table then your table rows are causing this issue: While in the main report the sub-report may look like as small rectangle. the actual subreport is containing a table which can grow according the amount of data.
the solution is simple:
if you know the max records of the table, and you manage to fit the size of the subreport - you are good.
additional option is to define the
canGrow
property of the subreport (in the main report)