My opinion... If you provide correct light, enough water and fertilizer, then the new bulbs will grow large and plump, no matter what the age of the plant. Though my 2 Oncidium types did make smaller bulbs when they got way too tight in their pots. I repotted them, and then they were growing normally again
Though when the plant gets to be quite old and large though, there may be problems. Big plant= big pot, and a big pot doesn't always dry very quickly or evenly depending on the conditions. In that case roots tend to die off in the stale medium. Also, on older plants the oldest bulbs die off some number of years. In both those cases the usual course of action is to divide the plant, which rejuvenates it and it'll grow very happily again.