I've read that somewhere - is it true? They explained that because you have a lack of energy when you don't get enough iron, you are likely to eat more so you can get some energy from food. What do you think?

Doesn't sound very convincing to me - weight gain isn't one of the signs of anaemia, if anything people may have a loss of appetite.
& it's not exactly energy you're not getting from the iron in iron-deficiency anaemia (bear in mind there are lots of other causes of anaemia!) but a component of haemoglobin which helps to carry the oxygen. Increasing other food wouldn't alter this. Maybe it happens in a few people, or people think 'I'm tired - i should eat more' but....

i'm anaemic and anaemia it didn't make me put on weight. before i got treated for it, i was tired all the time and i wasn't bothered with food at all.

If there's any weight gain associated with anemia I would imagine it would be from being too tired to get much exercise. Either way, I never had a problem when I had iron deficient anemia.

if it did occur, i think this would be the reason.

it's not a common symptom of anamia, it doesn't feature in any medical textbooks as a sign.

in fact a lot of conditions that cause anaemia might also cause weight loss, things like bowel problems/coeliac disease