Well if you were to use the larger dildos on a regular basis it would definitely adjust to that size and stay there until you stop using it. It isn't so much as getting stretched wider etc in the scenario of a woman getting looser that's more with age. think of it as the elastic waste band in underwear some pants etc now stretch that over and over what happens? This takes years and years though lol.
In your case if this toy is significantly larger than your partner then quite frankly constant use of it will make you accommodating for that size and he will be losing out. Hate to say less tight cause that's not whats happening there is more of adapting to larger size. If you do not use everyday for instance would be fine will tighten back up specially with the exercises.