What is the reason for Jiraiya have toad features while using Sage Mode?
Some people say that is because he was unable to perfectly balance his own chakra with natural energy but others say that is because his reserves of chakra were not big enough to prevent it.
To prevent gaining toad features, the user must have huge reserves of chakra to counteract the effects of senjutsu (toad features)? Or just has to be able to perfectly balance his own chakra with natural energy?
Once again, I'm not the best with these types of questions, but my personal guess is that it's both.
Fukasaku explains that, in order to acquire sage mode, you need to balance your own chakra with nature energy. If you gather too much nature energy, you will turn into a frog. Even if the scales are just ever so slightly off, the user will gain toad-like features, such is the case with Jiraiya.
But then Fukasaku also explains that without enormous chakra reserves, there is no point in training for sage mode. The nature energy would just overwhelm the user in an instant.
So what is it? Did Jiraiya fail because of poor control or insufficient chakra reserves? I think we need to differentiate between training and mastery.
First of all, nature energy is literally everywhere except it doesn't enter a user's body unless they actively draw from it (compare chapter #409, page 13-15). Fukasaku goes even further and tells Naruto how, besides collecting too much, you can also collect too little nature energy. All of this strongly suggests that the user can limit and stop the influx of nature energy.
Secondly, when Fukasaku explains that large chakra reserves are a prerequisite, he does so not in reference to sage mode itself but in reference to the training that precedes the true acquisition. As we see in the case of Naruto, even individuals with massive chakra reserves will start out struggling to counter-balance the massive influx of nature energy, both with and without the use of toad oil. This means that the training itself is extremely risky to everyone who dares.
In conclusion, large chakra reserves are only the prerequisite for training in the use of sage mode. But when it's about mastery, it all comes down to balancing.
So does Jiraiya just have poor control over nature energy and/or his own chakra? Not quite.
When balancing your own chakra with nature energy, there are mostly three different outcomes:
Too little nature energy (no sage chakra acquired)
Perfect ratio (sage chakra acquired)
Too much nature energy (user turns into a frog)
For obvious reasons, sage mode users will try to hit that perfect ratio, as both the "too little" and the "too much" ends of the spectrum lead to unfavorable results. However, for users like Jiraiya, who struggle to maintain the perfect ratio, there is also an acceptable alternative by drawing in just a little too much nature energy, hitting a nearly perfect ratio. It does have reversible drawbacks (gaining toad features) but it gets the job done and expands the margin of error for the user by not confining them to a metaphorical tightrope.
Now, Naruto, of course, is not forced to make use of the imperfect ratio. This is where my personal theory comes into play: I believe that this difference between master and student is primarily caused by the size of each of their respective chakra pools. With a larger chakra pool, minor fluctuations of influx may not have as much of an effect as they do on smaller chakra pools, giving them greater error margins. If you need a mental image, imagine seasoning your food. The bigger a soup, the less of an effect a spoon of salt will have on the taste.
This would also explain why sage mode training is better suited for individuals with large chakra reserves. Since the early training stages are the most dangerous (as the user still has no control over nature energy), individuals with larger chakra reserves (and therefore bigger margins for error) are less likely to turn into frogs right at the start. Their large chakra reserves are like metaphorical support wheels for learning how to ride a bike.
So yeah, if Jiraiya had a larger chakra pool, he'd probably have an easier time maintaining the perfect ratio for sage mode. Alternatively, if Jiraiya were to train more/harder, he wouldn't need a larger chakra pool. It's not like he physically cannot achieve a perfect ratio, it's just that it'd be harder for him to maintain a perfect balance than it was for Naruto.
Anyway, please keep in mind that this is my theory rather than proven fact.