1. Architecture: Docker Compose is a tool that allows you to define and run multiple containers, with the option to specify dependencies and network configurations within a single host machine. On the other hand, k8s-sidecar-injector is a Kubernetes admission controller, which injects additional containers (sidecars) into existing pods, enabling the sharing of resources and functionalities.

2. Scope: Docker Compose is primarily designed for managing containers on a single host machine, making it suitable for local development and testing. Conversely, k8s-sidecar-injector operates within a Kubernetes cluster, enabling the orchestration and scaling of containers across multiple nodes.

3. Compatibility: Docker Compose is compatible with both Docker Swarm and Kubernetes, but it does not have native integration with Kubernetes. In contrast, k8s-sidecar-injector is specifically designed for Kubernetes and leverages its API to dynamically inject sidecar containers into pods.

4. Functionality: Docker Compose provides a simple and straightforward way to define and manage containers, networks, and volumes using a YAML file. It offers features like service discovery, load balancing, and environment variables management. On the other hand, k8s-sidecar-injector focuses on extending the functionality of existing pods by injecting additional containers to enhance their capabilities, such as logging, authentication, or monitoring.

5. Deployment and Scalability: Docker Compose is suitable for small-scale deployments where a single host machine can handle the workload. It does not provide built-in mechanisms for scaling containers across multiple hosts. In contrast, k8s-sidecar-injector leverages the scaling capabilities of Kubernetes, allowing you to horizontally scale containers and handle load balancing automatically.

6. Maturity and Community Support: Docker Compose has been around for a longer time and has a large community of users and contributors. It has matured over the years, making it stable and reliable. However, the k8s-sidecar-injector is a specific tool built for Kubernetes and may have a smaller community but benefits from the active development and support of the Kubernetes community.

In summary, Docker Compose is focused on managing containers on a single host machine, providing a simple and convenient way to define their configuration and dependencies, while k8s-sidecar-injector extends the functionality of Kubernetes pods by injecting additional containers to enhance their capabilities within a cluster environment.