The structure of the fee system in Bitcoin is very dynamic... and generally works on a "supply and demand" basis.
The "supply" is the amount of space available in a block. In this case it is approximately 1meg (it's slightly more now that we have SegWit, but 1 meg is a nice round number for our example)... the "demand" is the number of people who want to have transactions confirmed, and how "big" their transactions are. Note that for calculating fees... the "size" of your transaction is the "data size" of your transaction, measured in bytes... it has NOTHING to do with the $ value or the total amount of BTC you are sending...
Now, given that we only have we only have 1 meg of space... or 1,000,000 bytes... it should be apparent, that when demand is high like it is now (we have something like 220+ megs of unconfirmed transactions... or 220,000,000+ bytes of data)... there are a LOT more transactions trying to fit into the next 1,000,000 block, than can actually fit into that block... So, the miners, generally prioritise which transactions to put into a block based on the ratio of "fee paid to transaction data size"... this is why it is important to look at the "fee RATE" (measured in sats/byte) as opposed to simply the total fee paid (ie. 0.004 BTC).
For instance, imagine a transaction of 10,000 bytes (which is effectively 1% of the total size of a block)... if it paid 0.004 BTC fee... it's effective fee rate would be 400,000 / 10,000 = 40 sats/byte. However, a transaction of only 226 bytes paying that same fee of 0.004 BTC would have an effective fee rate of 400,000 / 226 = ~1770 sats/byte!
It's fairly obvious which one the miners will take first!
So, if you have collect a LOT of tiny amounts of BTC (ie. "dust"... from activities like cloud mining, or faucet collecting etc)... you will likely have a LOT of inputs to your transaction if you try to send your BTC out. As we can see from the formula for calculating transaction size, the "data size" of your transaction is MOSTLY affected by the number of inputs (148 bytes per input vs. 34 bytes per output)... so a LOT of inputs = a LOT of data = a LARGE fee.
Now, on the flipside, when there is little demand (ie. previous times where number of unconfirmed transactions was less 10,000)... miners will generally take whatever transactions they can to fill blocks and get some fees... so prices drop.
For now, we're in a high demand, low supply cycle... fees are ridiculously high... If you've been collecting lots of smaller inputs, attempting to send them now is going to result in large fees due to the large data size of your transaction. You're probably better waiting until fees come back down to a lower level.
You always have the option of using a wallet that will allow you to specify your own custom fees... however the risk of doing that is you'll end up with the other 100,000+ unconfirmed transactions that are "stuck" because the miners are ignoring your low fee transaction in preference to the ones paying higher fees.
