Why have all the Earthlings so strong? It's not like they were ever implied to make big gains training with Kaio.
So remember, one saga effects another. So if we know the kids end up Cell Jr level at ToP, then we sometimes have to work backwards to account for everything.
The humans end up quite strong during the Androids Saga. Gohan + Krillen + Tien should more or less equal a Suppressed Piccolo. A Suppressed Piccolo who is weaker than the Base Saiyans, but still should be a respectable level. So the humans are making up quite a large percentage of what a Base Saiyan looks like in the Androids Saga. This no doubt is a very high level.
So going back to your question now. What I try to do is remain in the rational. Gohan no doubt has greater potential then the humans, right? So, his power should surpass the humans ability to gain power at this point of the story. So for me to get the humans to such a high level, I need to be wary of just how much power they are gaining compared to Gohan. I have Gohan getting 15x stronger over the 3 years, no doubt some of his gains come from now training over the last two years, but still, I think Gohan should gain just as much as the humans at worst and even more if possible Gohan's rate of progress over the Androids Saga was 15x. I deemed 10x for the humans to be a fair range and so to get the humans in line to be prepared to make a 10x jump instead of a 20x jump or even more, I have to start setting things up all the way back going on to the events that occurred on Namek. So that means making Krillen as strong as possible, which for me is 200,000. Then, despite nothing concrete in the Trunks Saga, I opted to give Krillen a tremendous boost as Piccolo, Vegeta, and Goku are suggested to have gotten, to have him in prime position to make his 10x jump for the Androids Saga. Then I just have Tien and Yamcha keep up with Krillen. Logic has never determine power, only plot, but when I can use logic I opt to do so in a way that seems most reasonable.
Why is android 19 that strong? Even without absorptions?
Android 19 is that strong due to the feats he showcased against a Super Saiyan that's implied to be beyond Yardrat Goku by Tien. Remember at this point of the game, power chains are extremely necessary since we don't have the help of power levels anymore. Now while Goku is no doubt severely dominant over 19, if you look closely to the key details, he's not quite as superior as Vegeta is over 19 Post Absorption.
This creates a power chain that looks like this...
(Vegeta > 19 Post) > (Goku > 19)
So this limits the distant we can have Goku > 19 right off the bat. Now combine it with the other power chain we established based on Tien's commentary.
Sick Goku > Yardrat Goku
Now don't forget, we actually have quite a lot of data from the Cell Saga from all of the tanks or failed tanks.
Tier 1: (Semi-Perfect Cell > Android 16) ~ (Perfect Cell > Super Vegeta)
Tier 2: (Super Saiyan 2 Gohan > Perfect Cell)
Tier 3: (Vegeta > 19 Post) ~ (Vegeta > Semi-Perfect Cell)
With these established tiers, we have now created limitations for a lot of our power gaps that must play well with these rules.
Then we have the assumption of Kamiccolo being 2x Piccolo as well.
There's a bit more stuff that goes on to making all of this play together nicely as well, but I'm sure this is a bit much to take in at once. So I'll cut it short here.
Even gaining a lot Vegeta said they weren't as bad as Trunks told them. They should be below the Trunks who killed Freeza.
Most people forget what the Dragon Team is expecting. They are expecting invincible, unbeatable Androids. When Androids 19 and 20 roll around with a measly power only around Yardrat Goku, they are able to tell quite quickly that something is off.
Freeza is seen as a big deal, Trunks killing Freeza weighs more for Kami than mentioning Piccolo defeating #20. This shows Trunks from that time and Freeza are likely stronger than Piccolo and the fake androids.
This is a similar argument to Freeza saying he didn't know anyone was stronger than Ginyu to go to meaning that Piccolo, Gohan, and Vegeta are weaker than Ginyu. Sometimes reputation means a lot more than anything. I'd say that we see this with Tao Pai Pai, Captain Ginyu, and Freeza as well where their reputation leads to a conversation that isn't perfectly in line with how we typically interpret statements poised to be looked at as power level statements.
Why are the base saiyans so strong? 18 was not using full power and kid trunks in base form needed defense from her attacks. She was playing around.
The Daizenshuu states that Base Trunks is on par with Android 18, the overall implications of Base Saiyan strength and so on lead us to believe that this is a reliable statement from the guidebook.
Android 18 is likely giving her best from what I can see and yet Trunks, despite his horrible conditions, is able to fend her off quite well.
The Base Saiyans are incredibly powerful, the adult Saiyans going even beyond a Post-RoSaT Super Namekian Piccolo.