Simply, the more services - which includes number of shards - that a node provides for the Radix network, the larger the potential earning base. Each node earns fees from any Temporal Proof's they create. The distribution of which nodes provide the temporal proofs is probabilistic, not resource intensive. In other words, you don't have to ‘mine a block’ to get TX fees, just confirm a transaction!
A machine will automatically maintain as many shards as are possible, given its resources; very small machines will run only a few shards, large machines many more. A full node can be run on as little as 16MB of Memory and a 100 Mhz processor. At the moment we run the test net with a 64 bit shard space, so finding light shards is trivial, even for a very heavy network.
For more information, see: https://papers.radixdlt.com/incentives/