Large diameter elements make the antenna resonant at a lower frequency.
Shorten the elements to resonate at the desired frequency. You'll
probably find that about 7.1" will work well in your case, though
without some test equipment you're likely never going to notice the
difference! Resonance is NOT required for proper operation as an
antenna. An advantage of larger diameter elements is that the useable
bandwidth increases, though again if you're using it on a single
frequency with narrow bandwidth, you'll never notice the effect.

Cheers,
Tom