I can understand feeling pressured- my whole family thinks I'm a little weird for keeping my 2.5 year old rear facing, but luckily my husband doesnt really care one way or the other so he backs me. Anyways my point is that if YOU feel hes more comfortable and you can prove that he's safer I would not listen to them. Hes YOUR son and you can put your foot down that as his mother its your job to keep him safe. I would show them/print off the NHTSA Website where it recommends he rear-face until 4 years old.
Now I'll also understand if its not a battle you want to wage in which case, back to your original question of a seat that is less bulky I would look into a SureRide for a tall child. Its another convertible so in a sense you would be "wasting" the rear-facing capabilites, but has some of the highest top straps so your son wont outgrow it before he gst to a good booster age. It also may be a plus that it doesnt turn into a booster- that way your family can't pressure you to booster him once he's 4 and 40 pounds because you would have to go out and buy another seat. It is not the most reclined seat so I would try and test it out at Babies R Us if possible beforehand, or if not, shop at a place that allows returns. It's also called the Titan 65 at walmart and sells for around $100.