Pokémon are increasing in strength because the developers are becoming more aware of what sort of base stat spreads are good for pokémon. They're quite well aware of which pokémon will be at OU level, as you can see from the different games in battle tower/frontier or in games such as PBR.
Also, a big factor in which pokémon are good and which aren't currently is the transition from Stat Exp. to EVs. When every stat could be maxed out, 100 in every base stat was amazing. Pokémon with low defences could still take a hit or two, some could even take super effective hits if they were 2x. Balanced stats were not that bad, obviously very focused stats were great too, as we saw with pokémon like Gengar and Alakazam being monsters in their own right. Now however, if your stats are balanced, you lack the focus in stats, making them generally inferior choices. With limited EVs, a Bellyzard can't take a hit at all, so it's a really risky glass cannon. (Not to mention SR nowadays).
With EVs came the need for a obvious focus in a pokémon's stats. Decent abilities go a long way too, but focus in base stats is key. High speed and attacking stats for sweepers. Sweepers don't need great defence, they're not meant to be hit. Likewise with walls and more bulky supporters, a good defensive typing with the right stats in HP and defence(s). If you look at the starters over the years, you see more focus in their base stats.
Charizard and Typhlosion well rounded, Blaziken had great attacking stats, Infernape has great attacking stats AND speed.
Likewise with Blastoise > Feraligatr > Swampert > Empoleon, you see the greater focus in stats, although it all really started after 3rd gen.
The point is, pokémon are being given better distributions which work well with the system of EVs. So yes, they are getting stronger, because the developers understand now what constitutes to a powerful pokémon.