In 'SOLID' the 'I' stands for Interface Segregation. The whole idea of this principle is to split large interfaces into smaller ones, more modular. In MVC service would normally have an interface that controller would rely upon. You don't want your controllers to know about concrete implementation of that service. Therefore, a bunch of services with one or two methods is a good thing to have.
Services normally return DTOs in large applications or domain models directly in smaller applications. DTOs normally means more work, but better separation of concerns. The typical flow is:
- Controller calls service
- Service returns an object (be it a DTO, domain model or something else)
- Controller maps DTO/domain model to a view model
Mapping can be done manually, but most developers prefer to use auto mapping framework such as Automapper because we don't like writing plumbing code and we can be quite lazy :-)
One of many discussions on stackoverflow regarding use of DTOs and domain models: https://stackoverflow.com/questions/2680071/dto-or-domain-model-object-in-the-view-layer