In my experience, the most common reason for this is blank lines that are within the used range within excel. Delete any blank rows below the range you have actually used (right click on the left and delete the whole rows, do not just clear contents), if that fails to reduce the size, use a macro like the following:
Sub DeleteBlankRows()
Cells.Select
Dim i As Long
With Application
.Calculation = xlCalculationManual
.ScreenUpdating = False
For i = Selection.Rows.Count To 1 Step -1
If WorksheetFunction.CountA(Selection.Rows(i)) = 0 Then
Selection.Rows(i).EntireRow.Delete
End If
Next i
.Calculation = xlCalculationAutomatic
.ScreenUpdating = True
End With
End Sub