The biggest guys in cycling, track sprinters, regularly hit 160rpm+. It's about efficiency and not putting too much pressure on your knees. Yes, it's harder at first if you're used to grinding out the big gears but your knees and joints will thank you in the long run. That and your overall fitness should improve (high reps and low weight, basically). Either concentrate on spinning or buy a cadence function computer and try and keep it 90-100rpm all the time.