The Helots outnumbered Spartans as much as 7:1. They were totally responsible for the Spartan state to function; full Spartans only engaged in military service and political office. Helots did all of the agriculture, the making of material goods, etc. So because the Spartans were so dependent on them, and because they were so outnumbered, it can be argued that the strict Spartan military lifestyle was not in preparation for conflicts abroad, but rather as a necessity to keeping their slaves subjugated and afraid of revolt--and there were a handful of revolts. To worsen the population disparity were the strict Spartan customs concerning sex and the regular practice of infanticide of children who did not meet the high Spartan standard.

So the answer is basically that, since the Spartans were so outnumbered and afraid of and yet dependent on their slaves, the Spartan slaves were treated worse. The examples I'll point to are:

1. That the Spartans declared war on the Helots every autumn, so killing them was an act of war (and a service to the state) in stead of a crime.

2. Part of the Spartan education was the practice of actually just going out and killing some Helots.

3. The Crypteia was a sort of "secret police" which was meant to terrify the Helots and keep them in check.

4. In the fourth book of Thucydides, the Spartans gathered the 2,000 strongest, bravest Helots on the pretext of their being used in the war against Athens, promising them freedom in return, but then killed them all.

I know of no such incidents in Athens.

Edit: I'll also add that the Helots were property of the state, whereas Athens' slaves were held by individuals. Accordingly, private property and abundant public property are treated differently. I would have a hard time proving this, but I feel comfortable saying that Athenians viewed their slaves as sub-citizens, and the Spartans viewed theirs as sub-humans.

Helots were more a kin to serfs then slaves as other have said.

Chattel slaves could be bought and sold. They were not allowed to live in family units, often did not know who their parents were, were not allowed to engage in any sexual activity other than that sanctioned by the master, and any offspring were automatically slaves as well. The master decided if a child would be allowed to live, and could sell the child whenever. Chattel slaves could not own property any earnings derived from their activities – prostitution,art agricultural labor – belonged to the master. They also had no legal rights, In Athens for example, slaves could be tortured in any legal suit against an Athenian citizen.

Helots, answered to the Spartan state as a whole and were required to work the land. They could not be bought or sold. They lived in family units, knew their parents, chose their wives, and raised their own children. They retained 50% of the fruits of their labor and could sell what they did not consume on the open market, while a Spartiate who tried to extract more than his proscribed share from the produce of his estate was subjected censure. Helots could could accumulate wealth and spend it as they pleased.

This site explains it well I think. http://elysiumgates.com/~helena/Economy.html