We're not making the gap bigger than what it is. At the beginning of the series, Laxus was decently more powerful than Erza. He would have soloed Phantom Lord, and that's not just him hyping himself up. Gajeel called him a monster and was scared at the prospect of him being involved in the Phantom Lord war, and Gajeel was the strongest in that guild other than Jose. He had the feats to back it up too. He basically 2 shotted Gajeel with a roar and a heavenward halberd, and then had enough power to cast Fairy Law. If Laxus had been around during Phantom Lord, he would have soloed Jose and Gajeel in combat alone, and then Fairy Law would have been overkill.
During Tenrou, it was Laxus' magic power that allowed Natsu to do everything he did against Hades. His magic power gave Natsu a power-up that was comparable to when he ate Etherion, and Natsu would have beaten Hades if not for the Grimoire Heart. Laxus could have just as easily kept his magic power for himself and kept fighting. We know this because despite giving Natsu all of his MP and then being hit by Hade's blast, Laxus was still able to get back up and punch Hades. He didn't have the guild mark either, so Tenrou tree didn't restore his MP.
During GMG, Mirajane commented about how she's uncertain if both herself and Erza combined can take down Jura. Laxus isn't uncertain. He knows he can solo Jura. Honestly, it's pretty clear that there was a signficiant gap between Laxus and Erza the start of the series. Erza may have closed the gap during the GMG arc by receiving 2nd origin, but Laxus still edges her out.
Erza herself thinks that Natsu is close to her level and can surpass her during Tenrou.