This might seem like a silly question and it might be the wrong thread for it, but I recently visited the National Museum of American History and I kept getting this weird feeling that all the period garments on display, with the exception of George Washington's suit, looked smaller than they should. Is it just me? Were people smaller back in the day? Did the clothes shrink during the preservation process? Did the clothes on display just come from smaller people rather than larger people?
This also kinda goes for the museum of Natural History in the taxidermy and bone exhibits, but presumably that's because either the specimens are just smaller animals (size of animals varies a lot), or I'm used to large than lofe presentations of these animals in films and on TV.
I can't tell you definitively why the clothing you saw looked small, but I can give a handful of reasons why you might have seen them that way.
People were a bit shorter in the past.
A bit. A small bit. In the second half of the eighteenth century (and through to the 1950s), the average American-born man was about 5'7"-5'9" tall; since most of the data on heights comes from military records, women's have been less studied, but the research that has been done shows an average height of 5'2"-5'3". Today, the average American man is 5'9"-5'10" and the average American woman is 5'3"-5'4". So that's not a significant difference, but it could play a slight part in your perception.
Ordinary people look short to you.
These days, particularly in America, we are steeped in media made with actors who are cast specifically to be attractive. One physical attribute generally considered attractive is height, and so we end up with men and women onscreen who are either fairly tall or whose height is bolstered with high heels/lifts or tricks of cinematography. As a result, you may unconsciously be expecting to see a taller figure presented to you.
The NMAH uses headless dress forms rather than mannequins.
The First Ladies exhibition - the bulk of the clothing on view at the NMAH - uses forms that in many cases don't even have necks. Other displayed clothing, such as in the Mad Men exhibitions, feature the same type of forms, with hats held up above the shoulders on thin metal rods. All of this can affect how you see the height of the "person". To add to that, the First Ladies exhibition largely consists of ankle- or floor-length formal gowns, and the long skirts can contribute to an impression of shortness.
(There's nothing about the preservation process that would shrink the clothing. Textile preservation largely consists of stitching backings or overlays onto the fabric, sewing loose beads into place, very gently vacuuming a piece or very gently washing it, maybe undoing alterations to return it to its original state.)
In addition to the answers already given, it's important to remember that the clothes which end up in museum collections are not always representative of the clothes worn by a wider population at any given historical period. Remember, textiles are fragile. They're sensitive to light, damp, moths, abrasion/tearing, and many other environmental risks. Most clothes just don't survive for long enough to make it into a modern museum (which is also why there are far fewer clothes on display from, say the 16th or 17th century, than the 19th).
That being the case, the garments that do end up in collections are usually those which have been (a) constructed from the most valuable fabrics, and (b) carefully treasured and passed down through families. For both of those reasons, therefore, the garments are likely to have been altered throughout their lifetimes, the original garment picked apart, and the fabric cut down and sewn up in the prevailing fashion. Because it's always easier to cut something down than to add fabric, the 'final' state of a garment is often very small, because over the years bits of fabric have been discarded and it has only managed to fit progressively smaller people.
Examples of re-made garments in museum collections include this dress in the Victoria and Albert Museum, where a 1770s silk has been remade into an 1840s evening dress, and this bodice in the Museum of London, which is in a 1780s pattern but made from a fabric dating to about 1760.