What does researching "Advanced Propulsion Tech I" and higher do in Master of Orion 1?

Advanced techs have two effects. They add to your end game score, and they raise your tech level in their field. Tech level is important because it effects ship part miniaturization. For every 10 levels you have in a tech field above the minimum required to use a part, they cost half as much money, and take up half as much space. Your tech level in a field is determined by taking the tech level of the most advanced technology you have researched in a field, and adding 1 for every less advanced technology researched in that field.

Source: http://www.gamefaqs.com/pc/564694-master-of-orion/faqs/1927

According to the manual:

Advances in technology will reduce the cost of building existing ships and missile bases. For every ten levels above the minimum required level of technology that you have, the cost of producing that device is halved...Although there are no new devices at higher than 50th level technology, you can still decrease the costs of producing existing technology.

Along with reduced cost, advances in technology also allows you to reduce the size of existing devices so that you can fit more on your starships. The mass of a device is decreased by 25% per 10 levels of technology above the minimum required level. Weapons, however, reduce by 50% for every 10 levels...Miniaturization works on computers, ECM jammers, armor, force fields, ship engines, weapons, and most special devices.