Okay, I think I finally see that you are:
the Panzer tanks developed in Germany in the 20th century. Note that 'Panzer' is short for 'Panzerkampfwagen' (meaning 'armoured fighting vehicle') - I will just refer to them as 'Panzers' throughout this write-up!
When I was one, I hid my species.
The Panzer I was a light tank produced in Nazi Germany in the 1930s, and was used in the invasions of Poland in 1939 and France in 1940. As the first model in the Panzer lineage it had no animal 'nickname' - unlike several of the later models (more on that to follow, below), so could be considered as having 'hidden its species'.
When I was two, I was still light.
The second Panzer model - Panzer II - was an upgraded version of the Panzer I, but was still classed as a light tank.
When I was three, I gained weight.
The Panzer III, however, was heavier (it 'gained weight') - a medium tank.
When I was four, I was many.
The Panzer IV was the most numerous German tank to be used in the Second World War - 8,553 of these were built ('many', indeed...).
When I was five, I was a kitty.
The Panzer V was the first to be given a nickname: the Panther (i.e. a type of cat).
When I was six, I was a bigger cat.
This was followed by the Panzer VI, which was known as the Tiger - a bigger cat.
I skipped seven, no time for that.
The Panzer VII - which was to be nicknamed 'Lion' - was only ever a design for a super-heavy tank. However, it never became a reality, skipped in favour of a new design by Porsche...
When I was eight, I became a plague.
...which was the Panzer VIII, nickname 'Maus' or 'Mouse' in English - an animal which is known to occur in plagues in some parts of the world.
I never reached nine, I'd say that's fake.
And that's where the Panzer lineage ends. the Panzer IX (and indeed X also) never actually existed - instead they were fake conceptual drawings printed in a German military magazine purely to deceive Allied intelligence!
To address the hints:
1. When I was eight, my name didn't reflect my size.
The Panzer VIII, or 'Mouse' was a super-heavy tank - far from being mouse-like!
2. I was the best when I was six.
This is probably one up for discussion, but some consider the Tiger heavy tank (Panzer VI) to be Germany's mightiest tank during the Second World War. (Although I have also seen arguments in favour of the Panther also...)
3. I never reached nine because my creator had perished.
No ninth Panzer model was ever actually built as by this point Nazi Germany had been vanquished.
4. I am a lineage of vehicles.
5. I am from a European country.