My brother LS-2725 is sewing perfectly on surface but has large loops underneath.

Contact Brother sewing website they have lols of helpful answers and videos. :-)

Machine incorrectly threaded. This is easy to do even with experienced sewers. Wrong needle, thread or material combination. Make sure the thread is not old and is the correct for the material and the size of needle Make sure the needle tension is correct for the material etc. :-)

Loops appear on the surface (top side) of the fabric?
Reduce the tension by turning the upper tension control dial to a lower number. If the loops still appear on the surface of the fabric after you adjust the upper tension, the bobbin thread may be too loose. In this case, turn the screw clockwise, being careful not to over-tighten the screw, approximately 30-45 degrees to increase bobbin tension. After you adjust the bobbin thread, adjust the upper thread to match the bobbin thread.
Make sure you reset before your next project.

1. Always make sure you are treading the sewing machine with the foot in the UP position.

2. Loops on bottom mean the top tension is too loose... tighten it.

check you top thread tension first
then if you still have loops check bobbin thread tension
also check your needle

Try some metal bobbins, #15 or SA 156. that seems to help. If, you don't have a manual go to brother.com and download one for free

If you threaded the machine with the presser foot UP and the thread from the bobbin is feeding from the spool in the correct direction and through the guides on the bobbin case and the loops are still forming, then the machine needs to go into the shop as the timing may need to be adjusted and this is best done by a sewing machine technician, preferably Brother.

loopy underneath means upper threading and/or tension wrong

This is an answer to another person with a similar complaint. Try the suggestion below, it may fix the problem.

From your description it appears that you have tried tension settings on the upper tension control adjustment knob without success.
Check that the bobin tension is adjusted correctly by taking out the bobin complete with the spool.
Pull the cotton thread and it should be a smooth light pressure if the tension is correct. If it is tight to pull or pulls freely then adjust the tension screw which is found on the outside of the bobbin. The screw is very small and requires a minature size screwdriver. The adjustment will only be a slight turn of the screw one way or the other to obtain the correct tension(depending if it was loose or tight).
Once this is set reinstall the bobin and tread up the machine, hopefully with the upper tension set at approx."2" the machine will sew correctly.

It sounds like your tensions are out of adjustment. Your upper tension should be between 3-5 and make sure the tension in your bobbin case is not too tight as well. Pull the thread through the bobbin case and make sure it comes out with a small amount of resistance. If no resistance, tighten the tension just a bit, if too much resistance, loosen it just a small bit at a time.