I don't know of any equipment or gems or runes you can have that increases the amount of rss you can send to another player.  Yes market level and boosts that you can apply that increase convoy capacity.  There are knowledges you can do in the economic tree (convoy capacity I and II) that increase the amount of rss you can send to another player.  Also increases in vip level allows more rss in convoys.

I wasn't familiar enough with this to answer the question ,but was intrigued by the response from CIM and LAIRD BRINNI.

However I needed to check something out first.

The hero doesn't appear to go out with the convoy and I have to guess that the shaman doesn't either,which means that runes and gems wont have any influence even if they existed.

The ones mentioned that do, surely will only affect bringing in the loot, not moving it on.

Also the use of the word capacity is significant where the questioner talks about moving the maximum quantity of resources.

I had a look at what is visible in my game, and the market does determine the maximum that you are allowed to send, i.e  presumably convoy capacity,but what maybe of importance to some players is how much your troops can actually move, which is determined by how many troops you have, and what you can do to improve their ability to carry  more resources.

See,The economic knowledge tree.

 I edited out what I first wrote because it appears to have changed since I first looked and just checked for other knowledge options. 

Of course there is also the 10% extra capacity from the game some days.