According to the MU-TH-UR 6000 Wikia entry, we have the following:
The MU-TH-UR 6000, known simply as "MOTHER" was a 182 model 2.1 terabyte AI Mainframe that served as the computer mainframe for the Nostromo, Mother auto-piloted the ship while the crew were in hypersleep and monitored their activities. Mother communicated with the crew through the intercom system or a special interface room only accessible to officer at the top of the chain of command.
Compare this to its entry for synthetics:
The Synthetic's mind is an integrated Carbon 60 processor with a processing speed of 10^15 floating point operations per second. Memory capacity includes 1 terabyte of fast cache buffer RAM and 1.2 Petabyte of non-volatile memory. The system is constructed around a very powerful heuristic logic driver, making decisions based upon imported sensory data, information drawn from experience and the android's vast inbuilt databases. Intuitive functions are derived from a suite of nested contextual and semantic programs linked by self-mapping loops of tangled hierarchies.
Basically, contained within one Synthetic is more processing power and ability than an entire spaceship AI.
I'm more than happy to admit that there's some inflation going on here (and the Synthetic entry isn't cited), but it has a point. In a universe of bulky mainframes and poorly synthesized speech and monochrome monitors, it seems impossible that anybody could build something as complex as a Synthetic.
So, question: Was there any official explanation in the first Alien for the disparity between the Nostromo and a Synthetic?