Impact of change of structure on table size

Hi..

I have created 2 tables
Emp_Int _Table
Emp_Id int not null,
Amount int
Clustered index on Emp_Id

Emp_Varchar_Table
Emp_Id int not null,
Amount varchar(15)
Non-Clustered index on Emp_Id

Entered 10 similar rows for both tables.
Checked the size occupied by each table and output is
name rows reserved data index_size unused
Emp_Varchar_Table 10 24 KB 8 KB 16 KB 0 KB
Emp_Int _Table 10 32 KB 8 KB 24 KB 0 KB

Now..
If I change the Amount field in Emp_Int _Table to varchar(15) or change Amount field in Emp_Varchar_Table to int, the size remains the same.

How the size remain same after changing the structure ?
What will be the impact of change in structure on Transaction logging and on Index ?

I will be grateful if some clarifies.

Thanks

10 rows in each table.
Emp_Id Amount
1 100
2 200
.
.
.
10 1000