Thats just the way it is once you cluster the planets they take the modifiers on the cluster level 1 and not its sub planet`s modifier
Cluster Lvl 1
Population growth + 10
Agriculture + 10
Mining + 10
However if you actually want a bigger land planet that is good for Agriculture i suggest you take time to explore a Ufertile and probable use an artifact to increase its land to the size you like
U.Fertile Detail
Fertile class planets consist of a moist atmosphere rich in carbon dioxide making is ideal for agriculture. The high amounts of carbon dioxide is poisonous to other life forms therefore restrict growth of occupants. This planet can not be clustered, infected or assimilated.
Land size ranges from 500 to 2000
Mine-able ore ranges from 0 to 50
Population growth -50
Agriculture + 250
Mining N/A