Well you can increase that space - there is a guide for repartitioning it - better still is the thread on symlinking several /usr directories into somewhere else. Its in the N900 section - will try find it shortly.

As for allowing us to choose where to install the app I can think of several reasons

1) Thats not how deb packages work (at least not with a vanilla install command)
2) You are expected to install optified packages - see above post on extras devel etc.