I really don't think you have anything to worry about. I know it's a cliche, but it's not the size of the boat, but the motion in the ocean that counts!
Bigger and harder are completely different things. This is a deeply unromantic analogy, but imagine for a second that you have an itch that you need to scratch. It doesn't make any difference whether you scratch it with a large hand or a small hand - the satisfaction is the same. What matters is that you find just the right spot and scratch it hard and fast! Sex is like that. When a woman asks you for it harder or faster, you're in the right place. She just needs more friction. Sometimes when we gals are building up to the big O, we need the movement to build up too, to help take us over the edge. Being larger wouldn't provide any more friction (our muscles grab you quite hard enough!). It's about technique, not size.
If you're really concerned about your wife's enjoyment of sex, why not devote a session especially to her pleasure, in which you make sure she orgasms without penetration? Also, there is a big difference for women between the different orgasms we feel from different types of sex - try worshipping her body and getting her really aroused in a selfless way, and you may find you reap massive rewards in the ways in which she returns the favour!
The other thing to say is that this may be more of a psychological thing for your wife, than a physical one. Harder sex can feel more engaged, and more passionate and romantic - psychologically it feels like you want us! Maybe she needs to feel wanted more often? Like men, women like to feel special and spoiled! Routine, commitments, work, stress - all these things can all interfere with time for sex, and sometimes you just need to make it special again for someone, to remind them how good it can be. Why not take a day off work, and spoil her a little - make her breakfast, buy her a little present, take her somewhere unusual and fun, have dinner. It doesn't have to be expensive, but it does need to be attentive and sweet and focused on her enjoyment.