How can I get better yield of small garden?

Make sure they have plenty of sun......don't know where you live?climate?growing conditions? what you are growing? so here's bunch of videos you can sort thru https://www.youtube.com/results?search_query=diy+better+yield+for+container+growing+up+

The bigger the mature plant, the bigger the pot is the best way to go. I use a lot of 24 inch pots from Home Depot pretty cheap. One tomato per pot. two or three peppers per pot, six leaf lettuce per pot and so on. I use ten to twelve inch pots for herbs and get huge plants with enough left over in the fall to freeze and last well into the spring. Make sure there is plenty of drainage holes in the pots and remove the lower dish so that the water does not keep the lower soil wet all the time. Anything planned in a pot will need to probably be watered more often than plants in the soil. In the heat of summer I can water my tomatoes up to two to three times a day to prevent the leaves from wilting. Since they are watered frequently they may need fertilizer on a regular basis and calcium supplements for tomatoes to prevent blossom end rot. I have had tomato plants in pots that have had a great yield, but the size of the tomatoes were smaller, but still just as good. Each time we get a new pot, we cut marine plywood to the size of the base and put castors on the wood so that the plants can be moved when needed, the larger pots get heavy real quickly. Do not foliar feed veggies, it will get you great green plants but little or no yield because the roots tell it to produce the flowers, not the leaves. Some calcium supplements are sprayed on the leaves for quicker absorption to the tomatoes. Always use good potting soil, I also add a little peat moss to the mix. I change about half of the soil each year and put what I take out up to the veggie garden at the top of the hills in the back yard (hubby can't make it up the hill, so he has his mini garden with his favorites by the house and garage).