This is the era of the 100kg+ inside centre with players like Hape of England, Jauzion of France, Sgarbi of Italy, Nonu and Williams of New Zealand, Morrison of Scotland and de Villiers of South Africa.
I see your point Bruce, it's helpful having a 100kg+ 12 as that is where the most traffic in a rugby game is usually directed. You can add to that list Jamie Roberts as well.
However having watched these players over the past few years I feel that I'm in a position to challenge you on this.
Shontayne Hape: Big direct 12 however he rarely uses his bulk and looks for his footwork to get on the inside shoulder and offload. It's his go to move and I bet most small stepping centres could do the same thing. They would also be selected over him if they had the "image" of a 6ft2 100kg centre. He is overrated and is often shown up defensively in Bath colours. Shocking kicker.
Jauzion: Great player, strong, direct, offloads well and is a good defender. However in his age he has also lost a bit of pace and is often exposed in the line by quicker players. A few years back I witnessed Topsy Ojo floating past him with ease. Still class but his days are numbered if he loses any more pace. Alright kicker as well.
Nonu: Again great player and I think he is the best 12 in world rugby at the moment. Big guy and good in defence but hes not just a straight up bosh kind of player. Has excellent footwork and speed and a good fend. Have no qualms about him and he is the ideal 12 apart from a lack of a kicking game. Limited Kicker.
SBW: Bit of a freak and looks to have the goods. Again a big guy and solid defender. As you have seen he runs straight and offloads and has a good fend. Limited kicker.
Morrison: Fairly average, good tackler and a straight runner. No kicking game not very creative and does a job (ish).
De Villiers: Not as dominant as he used to be, lost a bit of pace due to dodgy knee and age. Good player back in the day and was a dominant defender. Interception king and limited kicking.
Sgarbi: Seen him play a few times for italy and although he is a good defender he hasn't really done much in attack so far. Cant comment on his kicking. Limited playmaker.
A lot of these 12's are selected because they can crash it up and defend. However they don't offer a realistic line breaking threat (besides SBW and Nonu). Even Roberts who was a quality player a couple of years ago has a limited game. Stick a flanker on him and he does jack shit.
Australia have this sort of player in Tom Carter (don't take offence Bruce) he can run straight, tackles hard and is developing a passing game. However he doesn't have the incisiveness that Nonu, SBW, Hook, JOC (James O'Connor), Giteau (back in the day not now of course), Contepomi, Juan De Jongh, D'Arcy (a few years ago), Flutey and a few others have.
The average weight of these players combined is: 93.55 kgs
These are some of the best linebreakers in International rugby and only two of them are over 100kgs.