I bought this plant yesterday. The shopkeeper said it's a smaller variety of peace lily and will flower more usual than a bigger variety. Is this true? And according to him the reason why the leaves are not so green is because he put the plant in full sun all day long. How can i restore the dark green color that these leaves originally have? Will the flower/spathe be smaller in this variety? The leaves are much smaller and thinner than the images i see on google. Is there any way i can make the plant grow bigger leaves?
There are several varieties of Spathiphyllum or Peace Lily, and some are 'dwarf' varieties. Yours seems to have rippled leaves, so it might be the variety 'wallisi'. As for flowering more prolifically than its larger cousins, that's not true in my experience; in keeping with the overall size of the plant, the flowers will be shorter, and a little smaller than in a larger variety. Also note that not all peace lilies have really dark green leaves, some are much lighter green and with thinner leaves.
Your plant cannot be forced to grow larger than its genetics allow; if you want a larger variety, then look for the variety 'Mauna Lowa Supreme' or much larger than that, 'Sensation'. Information on some of the varieties, as well as a link to care of peace lilies, here https://homeguides.sfgate.com/types-peace-lilies-66499.html