No they don't. Them getting more than doubling in power in a few months does not happen everytime. Post HBTC Vegeta went in a second time and when he came out he was still below Goku who spent less than one year in it with Gohan. He got nothing out of it. Piccolo went in as well and was still so weak Vegeta berated him. Goku after fighting Tien to King Piccolo only went from a power level of 180 to 260. In the time skip to the Buu Saga, Vegeta only got SS2 and that was Vegeta's ASSJ form--> SS2 after 7 YEARS of training. It is not consistent and many of Goku's power increases are from Plot Armor.
Vegeta's in explained, he trained a year in the HBTC, not a few months. Goku caught up for no reason at all, aka Plot Armor power up. Goku didn't train after that. Vegeta should have stayed stronger, Goku suddenly becoming stronger despite Vegeta spending a year in the HBTC and Goku zero is never explained and quite silly.
- True but the same standard exists. Goku said they used more power. He never says the word transformation. KK uses more power like the transformations do, despite not being a transformation itself.
- And he was exhausted for a large part of it. Roshi even says that both SSB and KK both strain him and that by using both it was why he was so tired.
- Why wouldn't he, it drains Goku. Also, Vegeta's body has never experienced that strain. So he certainly isn't use to it.
- Because it uses more power. The power generated by SS3 Gotenks is less than KKx20 Gogeta. Gogeta is technically inexperienced with KK too since he has never used it in canon and Vegeta's half might make it more difficult to use.
- Goku's statement about more power reducing fusion timer.
Gogeta will be stronger with KKx20 but it wouldn't be by a huge amount. Vegeta doesn't have to win, he just has to not lose. All he has to do is bleed out the time.