Yes - my settlements are fairly small. In a first run through (I started a new character, when I was at level 28) I had 8 settlements, but none of them very big. Maximum 15 settlers and accordingly water + food. So around 30-40 defense maximum.
There is a problem when things become bigger. I am now level 30 and one settlement is at 17 settlers. I have 6 settlements now. I ignore Sanctuary, because I permanently receive quests from there, and I do not want this much settlements at the moment. Especially because their requests are purely random, so they keep sending you far to the south near the Glowing Sea. And I do not want settlements there at this time! It would be better if you could build settlements at your own speed and so, that they can support each other, i.e. beginning from Sanctuary and spreading out.
Despite that, sometimes the entire mechanism is unreliable, or buggy, or intentionally totally random with nasty side effects. For example, I was around level 12 or so, when I arrived at my new settlement Oberland Station (2 settlers, 3 water, 8 food). They had only one turret at this time (defense = 8). I was on my way north toward Red Rocket. Just as I passed an attack happened: 2 Radscorpions and one Legendary Glowing Radscopion!! A bit too much for one level 12 character and 2 settlers. I was lucky enough to have enough chems and a good weapon, and the attack happened directly under the turret. So I could win, but it was hard. Now - was this a settlement attack, or not? It probably was a random event I think, because the settlers thanked for the help 'against the Raiders'...
In another settlement a random event spot is near, and so often events happen. Mostly this are 3 dogs or 2-3 Raiders. My settlers (15-17) can defend themselves. I never had a defend quest from this settlement yet. But two times things were destroyed, turrets and plants. I suspect, that the game does not know if an attack is an settlement attack (planned by the system) or a random wandering in of some enemy NPCs. Both can only happen, if the cell is loaded, but if the player is far away, obviously every attack on settlers will destroy something, even if so weak, that they do not even enter the settlement boundaries. This is extremely unsatisfying and frustrating.
Others here report heavy attacks with higher player levels, and extremely high frequency of the attacks. This is clear, if you look at the mechanism, which (probably) was used.
Consider this: Lets say there is a probability for each settlement to be attacked. This probability is somehow calculated from food, water and defense (and maybe other values).
If the probability for a given settlement is 10% calculated every game day. This settlement would be attacked every 10 days average. If you have 30 settlements, then you will have 3 attacks per game day on average! If you have only one settlement, every 10 days. I think, that the attack probability should be somewhat mitigated if you have many settlements, or you will become tired of the defense quest rather fast...
Also - as very many things in this game - this is completely undocumented. It is unclear what to do, besides a strong defense, which will help, but not always. It is unclear what happens, and why: Red Rocket is sort of my home base: 0 Food, 0 Water, 16 defense (I built 2 turrets) and 0 settlers, 1 bed. Despite that it is attacked often (mostly 2-4 super mutants, once a Deathclaw). I never receive a quest and it seems to happen only when I am present. Could be wandering monsters. Does this count as an attack? I had several times damaged turrets. So what?
The entire system is undocumented and does not really work in a way you would expect. It seems totally random and illogical sometimes. And this is bad, since after a time players will not care, because you can change nothing. Not the ideal thing for a new game feature...
I really hope, that beth looks over this. I do not really trust in mods, they often do things I do not want...