Small People
Human artifacts are scaled for human bodies. If we were making a space colony or a starship for a mouse or a fly they could be much smaller, all other things being equal.
We already tend to employ smaller people for certain activities, such as professional flat race jockeys or as crew in Russian tanks. The latter give an interesting illustration of the benefits for the overall design. Western tank designers allow for the 95th percentile man (approximately 1.8metres) whereas Russians select tank crew of between 1.55 and 1.6 metres in height, and design their tanks accordingly. This allows lower hull height for the driver, lower height to turret roof and overall lower volume that has to be armoured, thus reducing weight which allows a smaller engine, and so on in a virtuous spiral of size reduction. Although other factors, such as reduced acceptable comfort and reduced crew numbers may also contribute, it is noteworthy that Russian tanks are typically smaller and lighter, (and cheaper to build and operate) even when crew numbers are taken into account (e.g. Russian T-90 is 8 tonnes lighter than French Leclerc, although both have 3 man crew). Another example would be the Japanese Type 61 tank, based on the American M47 but redesigned to suit the small stature of the typical Japanese (in the 1950s) and some 9 tonnes lighter (although again other factors probably contributed).
So, after that military digression, back to the point. If space vehicle personnel were selected, bred, reared or designed to be smaller, then we could build smaller, lighter, cheaper space vehicles. Lest this start to sound a bit mad scientist, Frankenstein or the like, I wish to emphasize that all such approaches should be voluntary, go through ethics committees and so on.
OK, so what could be done? Selection could be ethnic or, if that were considered unacceptable, of individuals who were small compared to their peers in similar environments. Having only women as crew may be an approach, since they tend to be smaller and lighter(although for long term colonization this would require perfection of parthenogenesis and even then would lose the long term genetic benefits of sexual reproduction). Hormonal dwarfing during childhood (or even by total protein and caloric restriction of diet) would leave the option of ready reversibility in succeeding generations if this were considered desirable, but this would present the quandary of informed consent from a minor and thus seems unlikely. Beyond that there are various genetic engineering scenarios and, I suppose some wild cyborg ideas, but I'll leave those to others. Generally, what seems most likely is that there could be benefits in affording and achieving space travel/colonization if we were to select from volunteers on the basis of restricted size and design vehicles/habitations accordingly.