It depends on period really.
4-5000 points, tank heavy will not buy much in the way of 500 point M1 in 2000, but will buy lots of 90 point Centurions in 1950. And I think the manual suggests 4-5000 points in the 70s as well, for beginers, and not tank heavy.
And many modern forces will be built to buy platoons or sections of MBT anyway, as they are so hideously expensive, and only to buy larger units on
huge amounts of free points. All armies will of course buy infantry on the round out, to spend the last scraps of points left over. And will buy leg infantry on the defence, bcause it is sensible for the AI to do so.
It also depends on the opposition - guerilla armies will not have any useful "tank heavy" option really. You won't see the Mujadeen or VC taking more than a few extra armed jeep things or light tanks, if they have them. It also depends on the opposition pairing - if
you have the low-tech infantry army, the AI pick may well be a "light" pick in that case as being the sensible mix against such (UK AI pick vs Argentina in the Falklands I think would be such a case).
"Tank Heavy" therefore usually is used in conjunction with
large points allocation (for the era), and on an army that has armour as an historical option. Because you want loads of expensive toys, you need to give the AI the means to pay for them.
Which army do you have a problem with the AI not picking enough tanks armour heavy, and at what date?. That sort of information makes figuring things out
much easier
Cheers
Andy