The fact that Django comes with even more batteries included than Python itself does not mean you have to use each and every battery. And ignoring stuff you don't need is often easier than extending an even moderately complex 'middle ground framework' in a meaningful way without making a mess of it.
If I compare that to my own experience, that is exactly what I've started to do over time.
A thing I've become accustomed to is splitting my applications cleanly in 'front-end' (focus on interaction with humans) and 'backend' (focus on business logic, (persistent) state and interaction with other applications/machines).
For front-end applications I've no real need for a number of things Django offers (ORM, authentication/authorization mostly). For building back-end applications Django has even more bits I do not really need.
This lead to a situation where I asked myself 'what does Django still offer me? Is it not easier to use a more limited framework for either use case or gobble together my own frameworks from various bits and pieces?'.
For front-end applications I could've (and have tried to) build a framework-to-fit from various bits and pieces such as cherrypy, jinja for templating, babel for i18n, routes for routing, etc. However creating a solid working framework this way, with all bits working together nicely - and keeping up to date with the development of its various bits and pieces - is a lot of work. Just using Django an ignoring the bits I don't need turned out to be far easier, leaving more time spending on my actual problem.
And for building my backend-applications I don't need all the stuff listed above - I simply need a robust and reasonable fast networked server which can host my business logic. The 'minimalist' frameworks then offer just what I need without getting in the way too much.
So in conclusion: if you want to build something that is mostly logic with a machine-machine interface, the minimalist frameworks offer a solid foundation and you have no real need for all the extras a 'mega framework' or even a 'middle ground framework' offers you. If you want to build something that has to interact with humans, you actually do need most of what the prevailing 'mega frameworks' offer you and leaving out the bits you don't want is easy. Hence there is no real need for 'middle ground' frameworks.
http://docs.pylonsproject.org/en/latest/docs/pyramid.html
The routing system in particular looks quite similar.
[2] http://packages.python.org/Flask-SQLAlchemy/
[3] http://packages.python.org/Flask-WTF/
[4] http://pypi.python.org/pypi?%3Aaction=search&term=flask&...