Adding tin will allow the mould to fill better to a point, and Heat will also be a part of the equation. Adding alloy weather it is tin, antimony or a combination will make for larger, but lighter bullets.
That said. If a guy is casting with pure and the mould is not filling out well due to not having it dialed in. If you add a pinch of tin the bullets can get heavier. Again that is because the mould is filling out to its max. I have found that moulds can be very finicky. Not every one casts the same.