There are different schools of thought on this. You should research on it and decide who you want to believe. One take is that the armpit is where leopard geckos store excess protein and other minerals, and tail is where they store excess fat. If you see these bubbles, it means your gecko is being a bit overfed. It's quite obvious as your gecko seems to have one of those balloon tails. The best thing to do is to simply cut down on feeding fatty feeders, and provide a better variety of low fat feeders. Also don't spoil your gecko too much by feeding them so much, as pet owners are often guilty of. If you cut down on what they eat, and start offering low fat feeders more often, you should see the bubbles go down slowly over time.
The armpit bubbles usually come and go often with seasonal changes. Like right now, the breeding season is over and females should be gorging as you may have noticed. All of my female breeders are pigging out. Their instinct is to eat as much as they can to replenish their weight loss from the breeding season, so they can better prepare for the next breeding season which is coming up in 4 short months. So for breeders, we often don't worry about these armpit bubbles as the bubbles go away during the breeding season as the females use up their body fat while fasting, and they use up excess nutrients when producing eggs. If you are a breeder, and you'll be using that female to breed soon, then the armpit pockets are of no concern. If you are a hobbyist, or you aren't going to be using that female for breeding purposes, then you should cut back on feeding, and start providing a more low fat variety diet.
The above are the findings and recommendations of vets and herpetologists, so again I'm just a messenger spreading these hard to find information. I say hard to find because some of the info I dug up a couple of years ago came from a forum that no longer exist. It was when a well known long time breeder consulted with a renowned herpetologist & vet on this and they came the conclusion above. People are welcome to challenge it and give their own take on it. But I find their explanation much better than folks randomly calling them calcium bubbles.