I would like to point out that we, Romanians, are usually referring to the age when we say about a person that it is big or small (despite the fact that we have distinctive adjectives for old (bătrân) and young (tânăr)). If we talk about an object or an animal, then we mean the size of it.
Not necessarily contradiction but difference. "Eu mănânc pâine iar el mănâncă brânză ". Both are used to link two sentences. You use "iar" or "și" when these are somehow related (like the examples above) and you only use "și" when they are not related ("el e mic și eu mănânc pâine ")
First of all allow me to congratulate you all for learning Romanian. It has a really tough grammar and sometimes may feel overwhelming, but if children can learn it then so can you. All you need is enough practice and with it experience.
The hard way to learn Romanian is by learning the rules first. The easy way is the way children learn. First get enough experience listening, reading, so you can feel the language. Then you can learn the rules.
Now, to get to the point, you felt very well about "iar". "Si" and "iar" are logically quite the same, but grammatically they are different. They are both coordinating conjunction but "si" is copulative and "iar" is adversative.
Your translation is correct. I just wanted to point out that if you encounter this type of comparison in a casual discussion, it could refer to age, not necessarily size. Even though I weight about 15kg more and I am taller than my sister, because she is 2 years older than me I would say "sora mea mai mare" ("my bigger sister" in direct translation) when talking about her. It's like G. Orwell's Big Brother - it refers to someone older/wiser, not someone physically bigger.