what makes linux so much different from dos is not design as much as it is philosophy, and that philosophy changes the design in a fundamental and unpredictable way that may take many years for the user of another system to appreciate. i've used ms-dos / win3.1 / caldera dos / win9x / winxp / giant bloated distros / mini linux distros for years. i started on tomsrtbt, which is floppy based, but didn't learn anything about linux that way. better to have dosbox and a linux term running side by side in a gui based distro, if you're getting familiar with linux, especially the prompt.
tc is much more simple than other gui based distros, the range of things you can do is incredibly wide for a distro of its size, and other distros that focus on doing everything with the gui add layers and layers of abstraction to simple tasks, even if it's all tucked under a reasonable and easy to use gui control. so the experience is one that while comfortable, is much more difficult to learn to automate (or appreciate). with tc not only is it easier to get under the hood, the parts under the hood are not as crowded. but then a few tasks are not half as intuitive to the end user.
for the kernel size, you have to think about what the dos kernel does vs the linux kernel. the dos kernel just gives you base memory access (there are 32 bit versions) and almost no support of hardware, which is interfaced with by drivers. in linux, much of this support is included with the kernel, and other modules can be added at runtime. the range of devices that linux can use is much wider than the range of devices that work in dos.
ultimately linux is about making everything an option. for a system where a million options are developed organically with little in the way of top-down organization or means of making them interoperable, it is surprisingly coherent, if less than dos. it's sort of like the difference between creationism and evolution- rarely, you end up with a tailbone or appendix that doesn't seem to do anything because it no longer does or more often, because it serves no purpose in your environment, but it's designed for a larger array of environments- it is amphibious. but for any system you can build with dos, it's probably easier (if you are already familiar) to build the same with linux, and far more things a linux system can easily be designed to do.
as for modularity, open source development for dos is so rare (the freedos project helps, but there's unproportionally, almost inexplicably more in the way of open source development for windows and linux) and there aren't so many reusable parts. as a dos user, i expected every application to either be be built statically or with dynamic components included in the same folder. in linux this is uneconomical, people use libraries and toolkits that become de facto standards so the installation procedure becomes a much different and more complicated one. tc has been great at streamlining that otherwise complicated process, the main issue with that right now is how quickly the collection of packages is evolving- the gtk2 upgrade has shaken things up, but they were impressively stable.