Eucharistic theology has never been discussed at the molecular level. As long as the consecrated host still retains the appearnce of bread, Jesus remains present in his entirety. It would be impossible, practically speaking to break off only a molecule of the host.
The teaching is that in the Eucharist, Jesus is neither multiplied nor divided. We don’t multiply his body by distributing numerous hosts, neither can he be divided by subdividing the host. The appearances are multiplied or divided, but Jesus remains one.
But to return to the OP’s question, in receiving only a tiny piece of the consecrated host, one still receives Jesus whole and entire.