Hi l have a lemon tree in a large pot which is over 2 yrs old from the ground it stands 2 mts high it is lovely and green has lots of flowers which in turn to a baby little green fruit after flowers has gone question is why the fruit doesn't get very big is it to young still.
Water and soil fertility are the first considerations.
From the photos the plant appears to have a severe nutrient deficiency, as evidenced by the chlorosis (yellowing) and mottled, spotty discoloration.
First be sure that the potted plant is getting enough water to facilitate absorption and uptake of available soil minerals.
Fertilize with a complete organic fertilizer.