All I can think of is
- that your sub report control is larger than it needs to be if your report is as short as it can be, or
- the report is wider than the sub report control. So if it doesn't fit, Access seems to reserve space for a scroll bar (in your case, horizontal one) even though it won't show.
If that doesn't help, maybe a pic of the report view and design view would help.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.