That's to be expected. Flask builds on top of Werkzeug, itsdangerous and blinker whereas Bottle reimplements part of what Flask and the dependency libraries provides itself. However for the vast majority of applications having strong foundations to build upon is the right thing to do.
Flask is awesome, great job!
The difference between Flask's and Bottle's ram usage will be negligible. Out of box, flask is a wrapper over werkzeug and ships with Jinja2. The main memory usage will come from other libraries and extensions, and your own application code.
you've figured that out based on a simple hello world, right?