Why the PSU should be oversized? In the end if my components consume (for example) to the maximum 250W at full load, the power supply will deliver 250W, the same thing will make even a 500W power supply, but a 650W will do "less work" and probably the fan will turn less quickly and consequently it will make less noise.
If you mean a purchase price discourse then, I give you reason, a 650W costs more than a 500W.
...or am I wrong?
Yes and no.
Without overclocking your expected gaming power draw should be around 200-220W at most (stress testing and synthetic benchmarks set aside): on the other hand, most of the time your upcoming build will be idling or facing low loads, so your average power draw should be much less, perhaps something between 40W and 60W.
At those loads an higher capacity PSU (to say, a 650W PSU vs a 400W PSU) will be comparatively less efficient, drawing more current than needed (leading to a slightly higher electricity bill on a year basis) and dumping more heat than necessary into the PSU itself (and then into the case: albeit we're talking of very few W).
With reference to reliability, at 40-60W a 400W PSU will run cooler, not hotter, due to the lower power conversion losses (usually efficiency quickly declines under 20% of the rated power with reference to 80Plus certified PSUs)
Equally, at 220W a 400W PSU will run cooler, not hotter, due to the lower power conversion losses (as it's about the 50% of the rated capacity, where efficiency maximize on 80Plus certified).
So, all in all, with an expected power draw of about 200-220W a good 400W PSU should last more, not less than an equally good 650W PSU.
With reference to quietness, it depends of the specific PSU series: sometimes an higher capacity PSU will run quieter, sometimes it does not. With reference to the proposed RMx it's partially true: the hybrid fan control starts usually around the 50% of nominal capacity, so an RM650x will run fanless up to about 330W (or more), while the smaller RM550x will run fanless up to about 280W (or more).
Said that, given an expected power draw of about 220W, it won't likely make any difference using the larger 650W, because both the PSUs will never start their fans, they both have a large headroom under any condition.
On the other hand, if you're going to compare different PSUs, then you have to take into account the relevant fan profiles. As a side note, if your expected power draw were much higher (to say, 400W, or even 500W) things would get worse for the larger RM650x, because its fan controller will speed up the fan with a steeper ratio than how much the RM550x does: so that up to 500W the RM650x would be likely a tiny bit noisier than the RM550x, and above 500W the RM650x would be noticeably noisier than the RM550x. Summarizing, there isn't a rule of thumb.
Add on top of that a good 400W PSU is cheaper than an equally good 650W one, usually, and you got some reasons to not overkill.