Are there any big spring-boot open source projects?

Here are some non pet store but a real world , non trivial , and open source application that use Spring Boot 2.

Thingsboard which is an IoT platform with the microservice architecture. Built with Spring Boot , Spring Security , Spring Data , Spring MVC etc.
Flowable is a business process engines that are based on Spring and have already upgrade to support Spring Boot 2.0
Spring Initializr is the backend web API that can quickly generate a sample spring-boot project. It is exactly the backend API that powered the famous start.spring.io.
Kafdrop is the web client that managing Kafka. Built with Spring Boot, Spring MVC, Freemarker etc.
Kafkawize is another web client that managing Kafka. Built with Spring Boot, Spring MVC, Spring Security,Spring Data JPA and Thymeleaf etc.
The backend of the Corona-Warn-App which is an app that helps trace infection chains of COVID-19 in Germany.Built with Spring Boot, Spring MVC, Spring Security,Spring Data JPA, Bean Validation etc.
CloudFoundry User Account and Authentication (UAA) Server is a multi tenant identity management service used in Cloud Foundry , but also available as a stand alone OAuth2 server. Built with Spring Boot , Spring MVC , Spring Security , Spring JDBC etc.

I know for example this two:

You can have a look at the Jhipster project.

It is a very nice project which helps to scaffold out SpringBoot applications.

You can get an idea of many of the best practices such as security, exception-handling, logging, task-execution, repositories etc.

In short, you can scaffold out applications based on the features that you need.