In a recent article in Antimicrobial Agents and Chemotherapy, Steven Projan points out that there is an apparent correlation between the size of a bacterium's genome and its penchant to display multidrug resistance phenotypes.
The smaller the genome the less likely it is that antibacterial resistance will emerge and disseminate within that species. What is proposed here is that, just as there is a continuum of genome sizes among bacteria, there is a continuum in the ability or propensity of a bacterium to become "multidrug resistant" and that continuum is reflected in the size of the genome.
The author informed me that he had intended to address the correlation between genome size and the ability to carry drug-resistance plasmids, but reviewers pointed out that the "data" is anecdotal. He writes: "There has been much futility in attempting to introduce plasmids into small genome organisms (as well as move in insertion sequences)."