I do not remember the specifics, probably Vovka can develop a bit what he said?
But, it should be near that: 1 horse can transport 1 troop + 1 crate at good speed.
So if you have say, 300 horses, you can transport 300 troops AND 300 crates, or you can transport only 300 troops, or only 300 crates (and 1 troop, say). But as soon as you have more crates and/or troops than horses, you will start to slow down.
If you have less than what you need, you slow down fast; if you have more you speed up a little bit but there is a near max limit.
Also a party of 1 troop + 1 horse will go much faster than a party of 10 000 troops and 10 000 horses. A troop has a weight that the horses only help reduce the speed malus, it adds up the bigger your army get.