Transact SQL :: Two Decimals Multiply Each Other Become Zero

Script as follows :

select cast( 0.0050000000 as decimal(38,23)) * cast(0.0000010000 as decimal(38,23))
select cast( 0.0050000000 as decimal(28,23)) * cast(0.0000010000 as decimal(28,23))
select cast( 0.01 as decimal(28,15)) * cast(0.0000000001 as decimal(28,15))
select cast( 0.01 as decimal(28,16)) * cast(0.0000000001 as decimal(28,16))

The result was following：

0.0000000--was zero
0.000000005000000000000000000
0.00000000000--was zero
0.0000000000010