So the goal here is to place the biggest Engine possible (a 190), without sacrificing too much Weight (an XL), maximising Weight savings (Endo-Steel Structure and Ferro Fibrous Armour), and carefully arranging your Armour to best effect.
(The exception to Ferro Fibrous Armour being the Pirates' Bane, which has no need of it.)
As a Locust you have incredibly small hit boxes on the Head and Arms - you can happily strip most of the Armour from these locations.
You don't have the Armour to take a hit if you 'balance' it, so you need to focus it, and use your large torso twist range to compensate.
By front loading the Armour, there at least you have the Armour of a 'mech 10-20T heavier.
The means your Rear Armour suffers - only the minimum 2/3/2 Armour required to take the occasional Laser hit, otherwise you'll be relying on the Front Armour, Torso Twist, outright speed and the map / cover.
Yes, the head only has 1 Armour, but you'll basically never be hit by more than the occasional rake of the Laser - you have 15HP plus that 1 Armour to survive any light brushes.
Anything heavier would probably take off a Torso anyway, and is
far more likely to hit said Torso, so getting hit in the head is a minor worry.
The Arms are also small, and not prone to being destroyed before you lose a Torso and die anyway, this combined with the Armour and Structure quirks found on the 'mech gives you sufficient part health to get by.
With 5 Armour an Arm (which results in 7.5 for most Locusts, 9 for the 3V), and 5HP, that gives you 12.5 (and 14 for the 3V) health each arm - enough to survive a solid hit from most single weapons, but again, anything bigger, would destroy a Torso anyway, and is more likely to hit said Torso anyway.
With all of this, as above, you get 7 Tons and 8 Slots free (14 with SHS).
This allows for some reasonable loadouts to go with your 170kph.
You do have two alternatives for Armour distribution (using the 1E as an example):
Base - Extra Armour
For some, the low armour goes too far against the grain, or their playstyle takes more risks, and they take more damage to the arms / head - for the cost of 1/2 ton, you can max the armour on the arms, and bring the head armour up to 5pts.
There's also
Base - Extra Armour, Balanced
While technically an option, it's a really bad idea - your largest Hit Boxes, your Torsos are no longer properly protected from any direction - just enough armour to survive
something, but nothing significant anywhere.