Don't know if I can answer your question or not. It's possible that over 12 years in a harsh space environment has tarnished the exterior but probably not enough to significantly reduce the luminosity. It's also possible that the solar panels don't reflect light like they used to. Another thought is that the ambient light levels (light pollution) may be worse now where you live than it was before. Another thought is that it might have something to do with the relative angles between the Hubble and the sun. That can vary considerably depending on where the HST is looking and perhaps with the time of year. If you happen to see the HST when it is aimed straight away from you, it'll look a lot smaller than if you see it more broadside. There are a lot of potential variables here.

Of course, it could also be your eyes. The last few years, my night vision has gone to hell. Getting old isn't for wimps.