Cookies really only spread out because of their fat content: when it gets warm it flows, and if it flows too much before the glutens start binding to give it structure, you get flat cookie. So, in this case, if the dough is colder at the start the fat stays stable longer, and lets the cookie set up.
You can try experimenting with your fats: maybe butter instead of crisco, or vice versa. Lot of vegan types will roll in some banana to counteract the lack of eggs and animal fats. Or you could maybe add some more egg?
Edit: If cooking longer softens the potatoes, then this isn't what's happening. In that case, well, you just need to cook longer. The main variable is probably temperature (maybe the pot isn't actually all hot for all 30-45 minutes), followed by variations in cut size and in the firmness of the original potatoes. But the rest could apply to some readers too!
I'm going to take a wild stab at this. It can explain this sort of thing, but it does depend on how you cook your stew. The following is a quote from On Food and Cooking (excellent book!), one of my favorite discoveries from reading through the sections on fruit and vegetables.
It turns out that in certain vegetables and fruits - including potatoes, sweet potatoes, beets, carrots, beans, cauliflower, tomatoes, cherries, apples - the usual softening during cooking can be reduced by a low-temperature precooking step. If preheated to 130-140F/55-60C for 20-30 minutes, these foods develop a persistent firmness that survives prolonged final cooking. ... Firm-able vegetables and fruits have an enzyme in their cell walls that becomes activated at around 50C (and inactivated above 70C), and alters the cell-wall pectins so that they're more easily cross-linked by calcium ions. At the same time, calcium ions are being released as the cell contents leak through damaged membranes, and they cross-link the pectin so that it will be much more resistant to removal or breakdown at boiling temperatures.
So, if you started the stew from cold (or drastically reduced the temperature by dumping in the sweet potatoes and other things) and had it on low heat, so that it took a while to come up to temperature, then your sweet potatoes may have been in that 50-70C range where the enzyme is active for a while. So maybe while your length of cooking time is consistent, your time before simmering isn't; that could depend on how cold the ingredients were before adding, the pot you used, whether you got exactly the same setting on your stove, the temperature in your home, and so on. (I know you said this isn't it, but having larger chunks of sweet potatoes could conceivably contribute to that too; the inside would take longer to heat up, so you could end up with a firm interior, and the softer part on the outside could get rubbed off.)
Sweet potatoes do also vary some in firmness, but certainly not by that much. It could still be a factor, though, in combination with the rest! In any case, if you want to avoid the firm potatoes, I'd try making sure to get everything heated up quickly.
Best Answer
First, it is the ingredients. For example, shortbread cookies don't have liquid, while other types do have liquid. This makes them quite different. Also, there might be ingredients you didn't pay much attention to (mono- and diglycerides, for example, which might have been present at the end as an E number only), but they still can change the mouthfeel a lot.
The second thing is the ratio of the ingredients. The same ingredients in a different ratio will work to give you a different texture.
The third thing is the process. Depending on the order of combining the ingredients, and the amount of mixing, and whether you chill them (and when, and for how long) you end up with completely different textures out of the same ingredients. It is like starting with carbon and ending up with graphite, graphene or diamonds, depending on how you do it.
The fourth thing is the baking process. You can change the temperature and time to achieve different final results.
I realize this list is very, very general. But listing all possible results which can be achieved at each step would make this a small book. Each of the four steps gives you a large number of parameters to change, which changes the end result too.