A smaller, darker first film that set a high mark. Sarah Connor/Ellen Ripley, a secondary character, is the only notable survivor. There is only one creature/terminator.
A larger, much more action oriented second film, that still holds onto the spirit. The secondary character who survived the previous film has been elevated to the lead. There is more than one creature/terminator.
A third film that breaks little ground in comparison to the previous two installments, but manages to hold onto what is probably the darkest ending in the series.
A fourth installment that is victim to cuts enforced by the studio, and the expanding ego of its leading actor/actress. Both will (probably) have much more time for their characters in the extended DVD versions. Both are (visually) well-made.
I think you can draw the comparisons, but some may be superficial in order to fit the analogy. There's plenty of ways in which they differ.