Are smaller door lock keys less secure?
The length of the key often corresponds to the number of pins in the lock. All else being equal, more pins makes it harder to pick the lock.

However, all else is seldom equal!

There are standards put out by ANSI/BHMA (the Builders Hardware Manufacturers Association) which allow residential locksets to be graded on their security, longevity, etc. Grade 1 is the best; Grade 3 is the worst (although it still meets minimum standards). These cover ease-of-picking, but also other aspects such as brute force (such as shearing the pins with a simple screwdriver, or just kicking the lock.)

There are other assorted standards out there. You generally have to buy the standards to read them, but many websites have summarized them. Here is a webpage from doorware.com that talks about the different grades and standards. You could ask your locksmith how yours are rated.

I spent a lot of time selecting locks for my new home. I became convinced that nobody picks locks anymore!  It's become too time consuming, it requires skill, you have to carry incriminating picks. Instead, a thief will generally kick in the door or break a window, get in and out quickly, and be gone before anyone can respond.

I'd recommend keeping the existing locks and putting the saved money towards an alarm system :)