Why my Onc.'s bulbs grow smaller and smaller?
 

It happens every times I grow Oncidium or any orchid with Oncidium parentage. The new 2nd(generation) bulbs grow smaller the than original, and the 3rd bulb(generation) smaller than the 2nd bulbs(generation).

Water by soaking or juz rinsing with water? How dry btw watering?

Dose it related to the water and fertilize habit?
Should I fertilize while new growth(bulbs) is growing or until the bulbs mature or after bloom?

I'm very confuse right now

Somebody correct me if I am wrong, but declining psuedobulbs usually points to lack of proper nutrients. Could be sufficient water (but you say soaking) so I suspect not enough nutrients. Since my water/fertilizer regimen involves daily water/fertilizer, I can only discuss that. I don't do the binge/starve method most seem to like.

Could be a lack of nutrients, like Ross said. I also dont like the binge/starve method (I never thought of it that way!) and water my guys every day. They are usually dry by the next day though because of fast draining medium and lots of air movement. If you dont have those conditions, watering everyday is not good. I have, however, seen plants that are in too dark of an environment put out smaller growths. Without the proper light, photosynthesis cannot occur and growth suffers. Most Oncidiums like pretty strong light. How high are your light levels? Any direct sun for your plant or are you growing inside?