I only have the anime to go on so maybe there's an explanation in the manga. It seems clear that Ichigo is stronger than Rukia was in episode 1 at the point of his first battle with Renji. In fact it seems like Ichigo may have been stronger as soon as he became a soul reaper, but based on Rukia's reaction to the stronger enemies up to that point (Menos Grande, and Grand Fisher) it seems she believes that a single shinigami could hope to win in cases where Ichigo does. This seems to imply that Ichigo is stronger than Rukia ever was at that point. Then Ichigo fights Renji and is at least closely matched. It's difficult to say that at the point where Ichigo gains the upper hand whether he is stronger or weaker than when he was fighting stronger enemies in the past but it does seem like Ichigo and Renji, at their first meeting, are at roughly the same strength which is greater than Rukia's at the beginning of the series.
But in episode 32 where Renji has his flashback to his time in the Rukon district and training to be a shinigami it seems clear that Rukia was always stronger than he was. So then how do we explain this discrepancy? Did Renji get significantly stronger in the few months Rukia was in the world of the living? Is there any particular reason for that? Or am I underestimating how strong Rukia was at the start?