why the big huge of steel floiats,and small coins sink?

Answer 1

The reason anything floats is that it displaces more surrounding fluid in weight than the object weighs itself. It displaces this fluid with volume. Doesn't matter whether or not we are talking ships or hot air balloons.

So, to make steel float, you just have to shape it into a shape that displaces more water (by weight) than the piece of steel weighs. This could be done by making it a hollow sphere, for example, where the sphere is large enough to displace an amount of water by weight that weighs more than the steel sphere weighs.

You can actually "weigh" something really heavy (like a loaded ship) in this manner. Just note the volume of water displaced by the extra load, and determine the weight of water for that volume. That's how much the cargo weighs.