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My thighs are around 22.5 inches /57cm when I measure them around the highest girth of the thighs, these can be considered average thigh circumfrence I believe, nothing too big nor too small.
But the thigh circumfrence around knee (about 2 to 3 inches above knee cap) is not that great (44cm), which makes them look below average I believe or not impressive.
How should I improve my thigh muscles
If you aren't already, make sure you include full range of motion squat patterns in your programming. I used to do leg extension variations until I was blue in the face to grow my VMO (the medial tear drop muscle you're probably most after). They helped a little (more on that in a sec), but it wasn't until I started doing deep squats that I really saw growth in my quads.
You don't have to do traditional barbell back squats, or even barbell front squats (a crowd favorite for quad growth) if you don't want to. The main thing is that you get as deep as you can go while staying pain free, and keeping full control throughout the entire movement (e.g., you see a lot of people in the gym going below parallel in their squat, but it often looks like: full control halfway down, then a big sudden drop, followed by a bounce out of the hole...this won't do anything for you other than jack your knees up).
Personally I got comfortable going deep with a lot of weight by doing [Hatfield squats][1] (named after the late great Fred Hatfield, aka Dr. Squat). You can get some gut wrenching sets in, but still maintain some decent control and safety due to the assistance of your upper body (when you need it). The only downside is you need a power rack and an SS bar (not every gym has these).
Leg presses, V Squat machines, belt squats (if you have back issues, or just really want to go buck wild on lower body), hack squat machines, and, of course, barbell squat variations all fit the bill; again, as long as you're hitting that full ROM.
Get your form down first, spend a few month building a good base, then occasionally try out some high rep widow maker sets (featured in popular programs like "Doggcrapp Training", "Mass Made Simple", and anything Tom Platz used to do). In short, start at your 10 RM (10 reps before form failure), and push yourself to do 20 (pause, breathe, hit another).
In terms of accessory work, leg extensions are great, but I highly recommend partial ROM for these. Position your legs in the machine, angle your toes out away from each other, bring the pad down about 15-20 degrees from the top end, then back up with a big squeeze at the top.
Many ways to go about building big quads, but that should be enough to get you started. Unfortunately, for folks like you and me who don't have quads naturally, we have to earn them with a puke bucket ;).
One and a 1/4 squats are known for blowing up the VMO (knee muscle) because you spend more time in the bottom position, requiring knee stabilization. Now it is argued that you cannot isolate different quad muscles as they work as a team. You can push more of the work towards different quad muscles such as VMO, but doing leg extensions with different angles is arguably inefficient because you cant "isolate" the muscle. You can also do various deep squats, and one legged squats, which shift focus a bit. The 1 and 1/4 is your best bet though, just dont make it your main squat of the week as its best used as a secondary exercise or a moderate weight and volume hypertrophy exercise. Make sure you are doing all the below as well to have a rounded quad workout:
