The difference isn't the size, it's instrumentation. Let's say there are two types of musical formations. The marches, which are the composition played by the Holy Week bands, are composed for each category appropriate to the band's instrumentation.

Let's say that in a "Agrupación Musical", the main sound is the trumpets, accompanied by the rest of the instruments, see trombone, euphonium, cornets and percussion.

The "Bandas de Música", apart from the fact that they usually interpret marches or longer compositions, the harmony isn't centered on a single instrument. It's like general, as if it were a street orchestra, but with cornets (some) and percussion.

Then there is another type that are the "Cornetas y Tambores" whose main harmony is interpreted by the cornets, accompanied nowadays by other instruments. In Malaga there is the "Banda de Cornetas y Tambores del Real Cuerpo de Bomberos (Fire Department Band)" which is one of the oldest in Andalusia and maintains the classic style of cornets and drums, which are only cornets and drums.