If you overload a washing machine it can cause physical problems, but it will also affect the efficiency of the wash. An overfilled washing machine can result in the laundry moving round in one large mass. The clothes will not agitate against each other and the washing machine detergent won't be able to circulate or dissolve properly. The washing will not be washed properly. With some liquid detergents there can also be potential serious consequences – Baby badly burned by washing machine detergent.
However, you do need to fill the drum in order to get the most economical use from your machine and to prevent under-loading, which is also bad. Under-loading the machine causes problems by making the weight of clothes more likely to accumulate on one side of the drum. This can make the load out of balance and cause violent banging on spin.
How much washing should I put in the drum when loading a washing machine?
Try to fill the drum for economy, but pat the clothes down lightly and make sure you can feel a reasonable gap between the top of the clothes and the top of the drum. Bear in mind that the drum may look really full, but once water comes in and the drum turns, many items will shrink in bulk. Leave a gap of approximately 3 or 4 inches between the top of the laundry and the top of the drum (roughly a hand's width). The laundry needs this space to fall into when the drum revolves.
Under loading the washing machine
Under loading the washing machine can also cause problems. In the old days it often resulted in violent out of balanced loads causing damage to the washing machine. These days under-loading is more likely to result in the washer simply refusing to spin so you end up with a finished cycle but wet laundry. If you have experienced some loads reaching the end of the cycle and not going into fast spin, then one possible explanation could be if you have under loaded it or are trying to wash and spin just one or two items. For a full explanation of the out of balance protection systems used by modern washing machines check out my article – Washing machine won’t spin just one item or very small loads. For a full explanation of the causes of a washing machine thumping around violently on spin read – Washing machine jumps around and bangs on spin