I'm the first born of four children... and trust me... my parents are VERY hard on me and my siblings can get away from so many silly things that I can't. I understand being the oldest child means big responsibility and role model to the younger ones, but it annoys me that even now when my siblings are much older, I'm still the one to have higher expectations...
I'm also the oldest of 4. I do recall my parents were tough on me, and my younger siblings seemed to have it easier. It was hard while I was going through it, but I realize that part of that is when you are the oldest, you are in essence, your parents' first "experiment" in parenting. You're the first to hit most milestones, so they test out their parenting plan on you which tends to be rather tough. When they see you survived, then they relax a bit for the next in line.
Also, each of us kids had very different personalities, so I know that played some part in how each of us were treated.
I was typically the first to be able to do things - it wasn't until high school where there were a couple times where my younger sister (a freshman) was allowed to do things I (as a senior) was not. By then, I was old enough to debate that logic with my parents, and when they couldn't defend the logic, they just fell back on the fact they were the parents. I was a good kid so I grumbled, but obeyed.
We're all adults and my other siblings are all married with kids (I'm still single and loving it) - and we all have fun joking about our time growing up - and comparing it to how my nieces/nephews are being raised.