大驚小怪 = passive: your or someone easily spooked/amazed/out of the blue/ by the situation or thing. 小題大作 = active: You or someone making a mountain out of a mole hill.

这是两个不同的成语，有各自的意思和用法，都很常用，通常是不可互换的。 大惊小怪，重点在于"惊"或者"怪"。比如明明很寻常的事情，你觉得很惊讶奇怪，别人就会说你，"你怎么这么大惊小怪的"，或者"这有什么值得大惊小怪的"。 小题大作，重点在于"作"。意思是，明明小事一桩，你却故意或者恶意把事情搞大，或者把事情弄得很严重。举个夸张的例子，比如说，这个小孩只是不小心踢了你一脚，你至于报警这么小题大作吗？

The two are both commonly used idioms. I find the translations for them in dictionaries are all but the same, and they actually have some meaning in common. And I think this is why they are confusing you. Although having some similar meaning, they are not used in the same situations: generally speaking, one of them used in a sentence cannot be replaced with another. When used in the same construct, there would be some difference in meaning. 大惊小怪 focuses on the behavior, manner, attitude, reaction， and 小题大作 on the action，performance. 小题大作 is more used to mean 'do something under the pretext of a trifle'. 不要大惊小怪的！ Do not make such a fuss. 不必大惊小怪 不要小题大作 Do try to do something else under the pretext of this trife. 不必小题大作

大驚小怪大多用於人的反應，而小題大作是用於事件或議題上

The first idiom may apply on people reaction most of the time

The second idiom may apply on the situation like some news

They are all common idioms, but actually the meaning are not the same

They all are very commonly used. and they are different meaning idioms using in different situations. 大驚小怪 when saying something or someone's event is tiny things but you look(act) too scary. 小題大作 is used to describe a small thing/topic you go(act) as a big motion. example： 公司虽然是个小公司，但不会倒闭的，你用不着大惊小怪。 你今天没有吃药，这是个小事情，请不要小题大作了。 sorry about my poor English. I hope it makes sense. hope you think it is little help.