Question: Many slot machines offer a disproportionately higher return on the jackpot combination when the maximum size bet is played. Do they also decrease the odds of hitting the jackpot combination when the maximum bet is played?
Additional information: My wife cannot afford to play the maximum bet, but when she hits the jackpot combinations, she thinks she missed out. I keep telling her that I believe the odds of hitting those combinations would significantly decrease if she paid the maximum, but she doesn’t believe me. In statistics terminology, it would make sense to maintain the same expected return (payout multiplied by probability), in order to obtain the same overall percent payback for different players (although I doubt they are required to maintain such consistency). Using this assumption, if the payout increases disproportionately relative to the size of the bet, the probability would have to decrease. This would also be a good way for the casino to dupe players into increasing their bets to the maximum.

Here's my interpretation of how slots work.
Slot machines are pegged to payout at a certain % by a computer chip. This percentage does not alter due to bet size at any time, so increasing your bet is essentially just losing your money faster. Higher denomination machines are typically programmed to payback a higher % than lower denomination ones, so that a $.25 machine will pay better than a $.05 machine which will pay better than a $.01 machine.
Regardless of bet size, even with the inflated jackpots for max bet, the expected payback remains the same on slot machines to the best of my knowledge. I don't think it's like video poker where betting max bet inflates jackpot size and increases expected value, but i'm not 100% sure.

So far I would say each of these posters is wrong or else I am misunderstanding what they are saying.
As to MAX COIN... Most, but not all, slot machines offer inducements for players to play max coin but read the payout table to be sure that final coin actually buys you something. However, whether you put in Minimum Coin, Maximum Coin or Something in between, once you hit that little red button... its all up to the random number generator and the random number generator just goes along and does its thing with no knowledge at all about how many coins are in there or whose player card it is or if they've been tipping the waitress or not. So the EVENT is not influenced by the number of coins, only the payment for that event.
Expected payback? You mean each and every press of the little red button has to be a certain expectation?
The general rule is that a higher denomination machine returns a greater percentage, yes, but going to max coin on a lower denomination has to be compared. Its amount bet times expectation.

The odds do not change when you bet max. I believe that some jurisdictions require identical odds. I've never, out of hundreds of games, seen a game that behaves differently.

Thank you all for your responses, but I am still not convinced that my theory is wrong. I may not have explained the problem adequately, and perhaps an example will be helpful. I could make up an example, but I would rather make it as realistic as possible. Next time I am in the casino, I will try to find a machine with the type of payouts I am talking about, and post an example.
One thing I will say now is that I think more of the average payout is related to the jackpots than you might think. For example, one local casino (Hollywood, FL) states that it pays out 94% of the bets wagered (i.e. a house advantage of 6%). I would conjecture that 5% or 10% if that payout is related to the jackpots. If it was really small – say 1% – then you would expect that, on the average, your credits would decrease by only $7 for every $100 you wager (such as 100 pulls on a $1 machine). But I will bet your average losses are much higher than that, unless of course, you hit a big win. This is because a significant portion of the 94% payback is related to the big wins.
I may be confusing the issue by using the word “jackpot”. I am not talking about the huge progressive jackpots of 100’s of thousands or millions that are rarely ever paid, but rather the smaller payouts of $1,000 to $10,000 that you might find on nickel and quarter machines. My wife, who only plays slots 5 to 10 hours a week, hits 3 or 4 of these “jackpots” every year. I know because I have to claim them on our tax return. Maybe I should call them 'big wins' rather than 'jackpots'?
Our next casino trip is later this week, so I should have a decent example to post sometime this weekend. If I am right about this, it is very important for players (such as my wife) to understand, so they don’t get duped into losing a lot of money, especially mine!