An Iron needle is long and skinny and if it is not too big you CAN float it by lowering it very slowly with a couple of toothpicks or similar tools.
I know your question says why does it sink and a much bigger piece of iron does not.
I just like the float a needle trick 🙂
Physics is a marvelous thing. With a longer than wide, flat and heavier than water item gets put into water it usually will sink. Why then does a huge heavier than water ship not (well under normal circumstances) sink?
It has to do with pressure and surface area. When you put a denser than water item on the water its mass with the aid of gravity causes it to go lower in the water. with the needle, if its mass is not too great, will not overcome the surface tension of the water and it will float. If it is too heavy once the down force overcomes surface tension the water rushes across the surface and the item goes down.
Ships and the like are not just flat, they are shaped like a V in the water and although they are far too heavy for surface tension, another force or two take over and it gets a little more complex and involves density. Density is Mass over volume I. E. kg/L. Water has a density of 1.0 kg per 1.0 Liter of water at STP. If something has less density than water it will float. Iron with a density of 7.874 g/cm3 is much heavier and yes, it should sink. Because of the vessels shape as it sinks into the water it has to push against the water and the water trying to remain where it was pushes back. the V shape of the vessel gets some support from the water pushing back. When the shape of the ship is such that it does not let the water in it eventually get enough push back from the water to hold it on the surface.
Huh? It comes back to the density thing and I suppose you can look at it several ways but I think if you take the shape of the boat and look at the volume, including the air inside the V part of the hull the density of the iron gets reduced by the air ( much much less dense than water or iron) incorporated into the hull shape and eventually the combined density gets to be less than the water and voila! she floats! Isn’t Physics Absolutely Marvelous?!! OOhhhhh and Planes and rockets and cars Oh My!! 🙂