I have some multi-varietal fruit trees (primarily apples and pears) that are starting to fruit in bunches of 3-4 per cluster. I am debating whether I should thin these clusters to 1-2 fruits (breaks my heart to lose 2 fruits per cluster) or let them grow to full size. What does the latter do to the quality of the fruit. The other related question is whether these fruits will thin on their own (survival of the fittest)?
There's not much to add to all the other answers, except to say that thinning also allows sunlight and air to reach more of the fruit, aiding good ripening/sweetening, and considerably lowers the risk of brown rot occurring from fruits touching each other if the weather is very wet. Younger trees producing heavy crops may be set back and not grow so well (as trees) for the following year too, and your trees are young enough for this to apply. The link below is from the RHS and is primarily aimed at growers in the UK, but is still relevant and includes information on how much to thin out
Trees left to their own devices will still produce fruit - they thin themselves naturally if there is too much fruit by 'June drop', but the tree only drops a number of fruits it cannot continue with without draining its resources, and this may not be sufficient for larger, sweeter fruits to develop. Untended trees will almost always perform biennial fruiting, with bumper crops one year and only one or two the following year. The incidence of brown rot is also greater in untended trees, and the fruits are likely to be smaller and less sweet. On the other hand, many gardeners don't bother with thinning, and do get a very reasonable crop of fruit some years, even if it is every other year rather than yearly, but thinning does increase the chance of larger, healthy and sweeter fruits yearly.
A picture would be nice. If the trees are young, they may not yet have an extensive enough root system and canopy to support all the fruit so you end up with lots of small fruit rather then larger specimens. And do you want growth wasted when they drop prematurely as the tree sheds those it can't support?
Another reason is that low lying branches laden with fruit can get diseases from contact with the ground, fungi, animals browsing etc. So you want to stop this too.
What "full size" IS depends on what you do or don't do (and/or what "June-drop" does or not do for you.) More smaller fruit - don't thin. Less larger fruit - do thin.
From the tree's point of view the smallest, seediest fruit that will still be considered edible by some animal who will transport the seeds elsewhere is "big enough."
If you thin the fruit, there are more resources to divide among fewer fruit, so each fruit gets more. This does not give you James's Giant Peach if you thin the whole tree down to a single fruit (unless you have a bag of magical green bugs, and are a character in a book), but is a good reason to thin, in general, if pollination/fruit set went well. The earlier you thin, the less wasted resources go with the culled fruit.
As well as the issue of small-sized fruit, another consequence of letting a tree over-crop by not thinning is that the tree may start a cycle of only producing a good crop once every two years.
This problem affects apples and pears more than other tree fruits. Some varieties are more prone to "biennial bearing" than others, but curing it is difficult - the most effective way is pruning the fruit buds before flowering, or removing the flowers before the fruit sets. This is much more tedious, and more time-critical, than thinning the fruit later in the year when you can easily see it. Once the fruit has set, the "trigger has been pulled" on the 2-year cycle so far as the tree's biochemistry is concerned.
So far as the tree is concerned, the two year cycle doesn't matter much. It only "needs" to produce one new tree from the thousands of fruits it will set over its natural life of 100 years or more. But that's not why you are growing fruit trees!