Belisarius

It's probably linked, people were always shorter because there wasn't enough food available, nor did people eat enough protein, vitamins and other nutrients.

Not only do people eat more than ever, certain animals like cows are given synthetic growth hormone, estrogen and testosterone; to make them grow faster, bigger and have more muscle.

And some studies have shown that we ingest some of those hormones aswel when we eat meat.

I think it's a mixture of reasons, populations that eat more animal products (i.e. the Dutch and most other Europeans) are taller than groups that consume less, or no, animal products (i.e. Indians). And combine that with the fact that we're simply eating more than ever and you see a population increase in height.

iirc human height was pretty stable until the 1800s, when technological advences meant that we got more food available.