There are several reasons to this : Though both are Namekians, there's a lot of difference between them. Piccolo is a warrior. He trains and meditates regularly. While Dende is the Guardian of earth and doesn't train at all. read more
Secondly, Piccolo has fused with Nail as well as Kami, so obviously his potential is much greater than Dende's. In fact, Piccolo is the strongest Namekian alive. I don't know if its true but I somewhere read that there are two groups in Namekians. read more
King Piccolo bestowed all his intent, willpower and strength into Jr, so instead of creating a child by using a bit of his strength that would grow up normally, he was using everything he had left to recreate himself as best he could. read more