Cute question. In the case that you gave (a monosyllable/bisyllable adj.+得多/多了）, there is almost no difference of meaning between them. Actually, ...多了 is the elliptical form of ...得多了, and the full form can be seen in some writing. The reason why the 'de' is ommitted in daily use is that modern Chinese has an inclination to make syllable sequence in double or foursome. As you read more and more, you'll find this insteresting law in Chinese, and this(prosodic or rhythmic laws) will explain many subtle phenomena in language.