1. Grid System: Bootstrap provides a more flexible and robust grid system compared to Milligram. It offers a range of responsive grid classes that allow for easy layout customization, whereas Milligram has a simpler grid system with fewer options.
2. Components and Features: Bootstrap comes with a wide range of pre-built components and features such as navigation bars, buttons, modals, and carousels. Milligram, on the other hand, offers a lightweight set of components and focuses more on minimalism, providing fewer out-of-the-box features.
3. Customization Options: Bootstrap allows for extensive customization with its numerous classes and themes. It offers a wide variety of customization options, which makes it easier to tailor the design to specific needs. Milligram, on the other hand, has a more opinionated and minimalistic approach with fewer customization options.
4. Design Philosophy: Bootstrap follows a "mobile-first" design philosophy, meaning it prioritizes mobile responsiveness by default. Milligram, on the other hand, leans towards a more traditional desktop-first approach, focusing on simplicity and readability.
5. File Size: Bootstrap, being a more feature-rich framework, has a larger file size. This can potentially impact the website's loading time, especially on slower networks. In contrast, Milligram is designed to be lightweight and minimal, resulting in a smaller file size and faster page loading.
6. Learning Curve: Bootstrap offers extensive documentation and a large community, making it easier for developers to find help and resources. It has a steeper learning curve due to its wide range of features and customization options. Milligram, on the other hand, has a simpler and more straightforward documentation, making it easier for beginners to grasp and work with.

In Summary, Bootstrap provides a more feature-rich and customizable experience with a larger learning curve, while Milligram focuses on minimalism and simplicity with a smaller file size and easier learning curve.