Like in most of the cases, the best variant is inbetween.
Both of these notions are detrimental for the trading account. Undertrading is inappropriate because by trading rarely with huge lots, a trader enlarges his risks. They do it in order to make trading profitable and make the profits more or less tangible. It doesn't make sense to do a deal once in month with the profit of $100. So, it makes more sense to do the trade with the profit of $5000 in the same period of time. However, as we all know, the more potential profit you have, the larger risks you run. In this situation, instead of gaining $5000, you can lose $3000 within a minute by sharp fluctuation of the price.
The other side of the coin is nowhere near to successful trading. The scalpers, for example, can make decades of deals within a trading session. I doubt that they are serious about market analysis, they look only at chart figures and several indicators and make faster decisions. However, not only are these decisions of poor quality, but it is also high psychological pressure. The more time you spend actively trading, the more psychological problems you are prone to like greed and fear. More often than not, traders trade on revenge: when they lose a series of trades, they are eager to make at least single successful trade.
So, these both variants are absolutely bad for trading, and I cannot really say, which is worse.