Security the bitcoin investment is the most crucial aspect of the business because without proper security, the whole effort of buying and holding can be brought to ruins. In other words, security of the asset is among the first things everyone should learn before getting started because scammers are becoming very smart and sophisticated these days. Unfortunately, many people still do not take the issue of security seriously just like you have rightly stated. This is the reason we still have people who store their assets in centralized exchanges, forgetting the importance of self-custody.

I don't think that the level of security an investor should employed should depend on the amount invested in bitcoin because every money is important. $100 to some people might be like $1 million for some people so both cases require maximum care to the security of the asset. The first is to learn about self custodian wallets and how not to share secret information to the public. Another is also not to click on random links to avoid phishing and other security threats. There are other measures the investor must employ to remain safe.

You are likely correct with any kind of suggestion that once a hacker (or person who gets access to another person's keys or wallet or account) has access, the amount may well still be taken whether it is small or large, yet I doubt that there is any real value to attempt to parse out the mentality of any kind of hacker or person who might get ahold of our keys.

It does seem important to spend more effort when more value is at stake or that the value of the loss is greater in the event that someone might get access to our keys or our account.

When a person is brand new to investing into bitcoin, maybe he is ONLY buying $100 or even $10 per week, so in the first few weeks, it could well be safe enough that the amount is saved on some account or even on a hot wallet that is on his phone, yet if the amount grows and grows and grows through ongoing buying and/or through possible BTC price appreciation, it likely becomes more and more important to figure out how to secure the coins in better ways. 

For example, it is difficult to imagine that a person might keep thousands or even $100s of thousands of dollars on his cell phone, unless there were maybe some special and/or limited purpose and time that the large amount would be on the phone.. So maybe a large transaction is going to be made, so the amount on the phone might only be there for a few hours or maybe one day or two at most.  Otherwise, maybe the amount on the phone would be no more than what a person might hold in cash in their physical wallet - which truly would vary depending on the kind of neighborhood that a person lives or hangs out in... some places are more dangerous than other places in regards to physical robberies or other kinds of ways a person might be insecure in their person and/or possessions.

It likely is a good practice to have extra security, like in the case that I mentioned in my above post where some value could go up 10x or even 70x in a relatively short period of time, then extra security might be preferable, yet also justifiable to spend some extra money or even create extra steps to access value... At the same time, it would not make a lot of sense to have such high level security for relatively small amounts, and sometimes normal people might end up making their coins so secure that they end up locking themselves out of their coins or even making it overly difficult or complicated that they may well end up losing their coins forever without anyone being able to help them to recover their coins, so there should be concerns about the negative sides to over security or even how it could become more difficult to pass down coins to heirs in event of death and/or permanent incapacitation.... which those surely are individual concerns about how much security should be taken to make sure that the keys/passwords are able to be passed down.. while potentially not giving access to the coins prior to they are authorized to be accessed.